More consistensy between reaction math

[yoelcortes]

This pull gives a bit more consistency between add and subtract methods in Reaction objects so that they work with zeros and None values. The following now works:

import thermosteam as tmo
tmo.settings.set_thermo(['H2O', 'H2', 'O2'], cache=True)
reaction = tmo.Reaction('2H2O -> 2H2 + O2', reactant='H2O', X=0.4)

# add
new_reaction = (reaction + None) # This was already added by a recent commit
assert new_reaction.X == reaction.X
new_reaction = (reaction + 0)
assert new_reaction.X == reaction.X

# sub
new_reaction = (reaction - None) 
assert new_reaction.X == reaction.X
new_reaction = (reaction - 0)
assert new_reaction.X == reaction.X

# iadd
reaction += 0
assert new_reaction.X == reaction.X
reaction += None
assert new_reaction.X == reaction.X

# isub
reaction -= 0
assert new_reaction.X == reaction.X
reaction -= None
assert new_reaction.X == reaction.X

Thanks!

[yoelcortes]

I went ahead and merged to mark long-term support for this math behavior with reaction objects and None types (which some old biorefineries rely on).