Refine EF to EFG conversion and handle errors from manually created EF files

[edwardchalstrey1]

Requested enahancement or feature

#77 introduced some AI-generated code for creating EFG format files from EF.

This should be more comprehensively tested.

Also, Bernhard notes that for manually created .ef files, the script:

it now detects symbolic payoffs but converts them to large numbers in the EFG file to indicate something amiss
a leaf with cosmetic {-}4~ is not parsed correctly to -4, but the { is converted to 9900 + ascii('{').

So we should handle this better if possible