Feature request: make pip installable with exclusion of fbprophet or any other pystan depender

[bramiozo]

Unfortunately, I have to work on an access-limited Windows machine because the hospital is still dominated by normies, that means I cannot do things like

• enjoy life
• pip install kats without depending on conda

Fortunately, the latter can be mitigated by excluding fbprophet from the install, I can do this locally of course but it would be nice if the windows-using public at large gets some light in that tunnel.