Miscellaneous operator support

Context

Add support for a number of operators: where, clip, squeeze, unsqueeze, reducesum, reducemean, slice, softplus, resize, topK...

Fixes Pad and Max operators

Tag status of operators' implementation in README

Modified files

• generator.py and export.py, corrections and additions to improve C literal generation;
• benchmark/ and tests/utils.py, to keep test results when required, and support optional parameters;
• operators.py, templates/, tests/operators/, and tests/strategies/nodes.py, to add the new operators' templates and test strategies;

TODO

Pending work is captured README Status section, as in fix the operators that fail the comparison to the Aidge CPU backend.