#1084 CIF code generator: automatically make the default code prefix a valid identifier

• Best to review per commit.
• The CIF code generator now automatically adapts the default code prefix to be a valid prefix, replacing unsupported characters by _, and adding a _ at the beginning if it starts with a digit. If the user specifies a custom code prefix, it is not automatically adapted and thus still a valid CIF identifier must be specified manually.

Closes #1084 (closed)