CIF code generator: automatically make the default code prefix a valid identifier

Currently, if you generate code for some-file.cif, you get an error:

ERROR: Code prefix "some-file" derived from the input filename of the CIF code generator is not a valid CIF identifier. Please provide a custom code prefix using the appropriate option.

It would be nicer if the code prefix is automatically adapted to be a valid prefix. If the user specifies a custom one, then it is not adapted, and it is still checked for being a valid identifier.

This solves the annoyance of having to rename the CIF input file or to specify a custom code prefix.