Not allowed rate names

[maurosilber]

Hi!

I was trying the new parameter expressions and there are some names that aren't allowed:

>>> rebop.Gillespie().add_reaction("infect", ["A"], ["B"])
ValueError: Rate expression not understood

The issue seems to be related to starting with inf, as using inXect as a parameter name works.

The same issue occurs with a parameter starting with nan.

Also, parameter names containing dots are not allowed: r.rate (while it is ok for species).