Problem with bioavalibility parameter in dose_absorb macro

[blakeaw]

I get an error when the input bioavailability (f=) to the dose_absorb macro is already a Parameter.

Offending code:

    if not isinstance(f, Parameter):
        F = Parameter("F_{0}_{1}".format(monomer_name, comp_name), f)
        params_created.add(F)
    pre_0_expr = Expression('precursor_0', (dose_expr * F))

When the argument f is a Parameter, the variable F is not created and the pre_0_expr expression errors out.

[blakeaw]

Fixed with PR #19