added constraint xi*gi = -xi*gi for skew symmetric class constraints

[ofercoq]

** @AdrienTaylor edition: Solves issue #110 ** Thanks @ofercoq !

[AdrienTaylor]

(with @bgoujaud @AdrienTaylor )
Todo:

• remove the change of "symmetry flag"
• instead, treat the case i=j directly with "add constraint from one list of points"

[bgoujaud]

Thanks a lot @ofercoq for raising this issue and fixing it!