Better errors for generated inits

[carrascomj]

Description

In the user_generated_inits file, if there is a row specifying conc_unbalanced parameters for balanced metabolites, maud will spit a wall of Stan errors:

...
Exception: ode_bdf_tol: ode parameters and data[14] is nan, but must be finite! (in '/home/georg/git/maud.git/master/src/maud/stan/model.stan', line 156, column 4 to line 203, column 39)
Exception: ode_bdf_tol: ode parameters and data[14] is nan, but must be finite! (in '/home/georg/git/maud.git/master/src/maud/stan/model.stan', line 156, column 4 to line 203, column 39)
Exception: ode_bdf_tol: ode parameters and data[14] is nan, but must be finite! (in '/home/georg/git/maud.git/master/src/maud/stan/model.stan', line 156, column 4 to line 203, column 39)
...

This can happen if/when the user changes a metabolite from unbalanced to balanced and forgets about the inits file.

Desired behavior

Some very light validation for the generated inits would be great to enhance UX.

[teddygroves]

yes this is definitely necessary, I get errors like that to do with inits all the time