double_deletion wrapper now calls speedup funcs
double_deletion wrapper now calls speedup funcs

For method="fba" the double_gene_deletion_fba and
double_reaction_deletion_fba functions are used. By default, the return
format is the same as previously expected, and the unmodified unit tests
still pass.

jeicher/cobrapy,jeicher/cobrapy,zakandrewking/cobrapy,aebrahim/cobrapy,zakandrewking/cobrapy,jerkos/cobrapy,JuBra/cobrapy,JuBra/cobrapy,aebrahim/cobrapy,jerkos/cobrapy