make reaction display prettier

Before:

>>> model.reactions.PGI.reaction
'1.0 g6p_c <=> 1.0 f6p_c'

Now:
>>> model.reactions.PGI.reaction
'g6p_c <=> f6p_c'
>>> model.reactions.ATPS4rpp.reaction
'4 h_p + adp_c + pi_c <=> 3 h_c + atp_c + h2o_c'
>>> model.reactions.CYTBDpp.reaction
'q8h2_c + 0.5 o2_c + 2 h_c --> q8_c + 2 h_p + h2o_c'