catching recompilation needed warning

[guiguem]

When a model has been compiled once with a different version of morpho/pyStan/python, the following error appears:

2018-02-14T19:47:49[WARNING ] pystan(369) -> No module named 'stanfit4anon_model_e8782a27df500e68b5c314485ddbc59a_1057640417982424343'
2018-02-14T19:47:49[WARNING ] pystan(370) -> Something went wrong while unpickling the StanModel. Consider recompiling.
2018-02-14T19:47:49[DEBUG   ] morpho(324) -> Using cached StanModel: ./morpho_test/cache/cached-model-e8782a27df500e68b5c314485ddbc59a.pkl
2018-02-14T19:47:49[INFO    ] morpho(331) -> Starting the sampling
2018-02-14T19:47:49[INFO    ] morpho(340) -> Parameters: 
data    [...]
chains  12
iter    4000
warmup  1000
thin    1
init    [...]
seed    36678357
algorithm       NUTS
n_jobs  2

2018-02-14T19:47:49[CRITICAL] morpho(648) -> Caught exception
Traceback (most recent call last):
  File "/build/bin/morpho", line 639, in <module>
    input_param, result = stan_cache(**(sa.gen_arg_dict()))
  File "/build/bin/morpho", line 342, in stan_cache
    return kwargs, sm.sampling(**kwargs)
  File "/build/lib/python3.6/site-packages/pystan/model.py", line 672, in sampling
    fit = self.fit_class(data, seed)
AttributeError: 'StanModel' object has no attribute 'fit_class'

We should try to catch it and force a recompilation of the model