Install Kats with CMDSTANPY instead of pystan

[lcoulet]

Hello, as for prophet it would more than nice to have the ability to avoid installing the pystan software.

e.g. for fbprophet we can proceed as follows:
CMDSTAN=/tmp/cmdstan-2.22.1 STAN_BACKEND=CMDSTANPY pip install prophet

Would that be possible to install Kats the same way ? And to document in the readme ?

[michaelbrundage]

I'm not familiar with CMDSTANPY, but if you want to experiment with this yourself, you could clone Kats, remove the pystan line from requirements.txt, and run pip install . in the root directory of Kats to install it. If CMDSTANPY is importable as import stan that should just drop-in work; otherwise, you would also need to modify the places in Kats where it has import stan. To fix #101 , I will make pystan an optional import.

[michaelbrundage]

After I landed f6d9d94 to fix issue #101, you should now be able to do this by first installing minimal Kats:

MINIMAL=1 pip install .

which will omit pystan and prophet, and then you're free to install optional other dependencies as desired.