Allow assigning input variables via SVG input mapping

Idea

See #203 (comment 273350) for the various ideas. This issue is to implement 'Idea U1'.

To do for a first version

• Update metamodel (including docs and diagrams), syntax, parser, type checker and pretty printer. And their tests. (!720 (merged))
• Prevent crashes in simulator and code generator for the new feature. (!720 (merged))
• Add simulator test for new feature (doesn't work yet). (!720 (merged))
• Update CIF to CIF SimplifyValues transformation. (!720 (merged))
• Make the new feature unsupported for the CIF simulator for now. (!825 (merged))
• Make the new feature unsupported for the CIF code generator for now (HTML output). (!826 (merged))
• Update the CIF merger. It should not be able to merge an SVG input mapping with an assignment, with another specification that provides a non-input variable to be merged with the input variable, as then the SVG input mapping would assign a non-input variable. (!833 (merged))
• Update CIF documentation (!837 (merged)):
  • In particular, the documentation about SVG (currently part of the simulator documentation) to document the new feature.
  • Check the entire simulator documentation, for whether additional updates are needed. Search for I/O decl, svg, input, event, addr, assign and update.
  • Check whether the CIF language tutorial's 'Variables overview' page needs to be updated. (#792 (closed))
• Fix the type checker unique assignments check for wrapping expressions. (#787 (closed), !824 (merged))
• Besides #787 (closed), the CifUpdateTypeChecker also does unwrapExpression. Will this break things as well? (!824 (merged))
• Fix CifAddressableUtils.getRefs and CifAddressableUtils.collectAddrVars to account for input variables as addressables. (!827 (merged))
• Update ElimIfUpdates to also eliminate if updates in SVG input mappings, and to support assignments to input variables. (!827 (merged))
• Make sure EdgeNoIfUpdatesCheck, EdgeNoMultiAssignCheck, and EdgeNoPartialVarAssignCheck really only consider updates on edges. Consider where they are used, and whether additional similar checks are needed for updates on SVG input declarations. Make sure the violation messages are correct. (!829 (merged))
• Update the documentation of the 'Simplify values' CIF to CIF transformations, to account for the fact that it also may simplify guards of if and elif updates on SVG input declarations. (!835 (merged))
• Fix the event disabler. It now assumes that the event of an SVG input declaration is non-null, which may no longer hold. See its collectSvgInputEventNames method. (!828 (merged))
• Fix metamodel documentation: (!836 (merged))
  • Assignment.addressableSyntax indicates addressables in CIF/SVG input mappings may be input variable references (non-wrapped) with projections. The non-wrapped part seems wrong.
  • Assignment.addressableSyntax indicates Adressables on edges may be discrete and continuous variable references (non-wrapped) with projections. The Adressables has a typo. There is a double space. I think we now do allow wrapped references, but we give a warning that it is convoluted.

What can be done later

• Improve metamodel documentation:
  • Texts like choose during simulation seems to suggest some features are only for simulation, which is not true. Make it about next transition or so.
• Improve checking of convoluted addressables in assignments on edges, now that we allow names there, not just identifiers. See #227 (comment 1987390) for an example.
• Support the new feature in the simulator.
  • Prerequisite: #778
  • Don't forget to update the CIF simulator documentation, including the documentation on SVG input and SVG output, if relevant. In particular the sentence 'The events that are coupled to visualization elements are called SVG interactive events or SVG input events.' may need updating.
  • Don't forget about the already extended svg_input test that has a Single update input (not yet implemented). comment that needs updating.
• Support the new feature in the code generator (HTML output).
• Update CIF examples to use the new feature.
• Link the CIF language tutorial 'Input variables' lesson to the SVG input mapping with updates documentation. Maybe explain it in the tutorial lesson a bit as well.
• Remove that the new feature is experimental (e.g., CIF simulator docs).