Warn the user if they are trying to save an unfit synthesizer

[npatki]

Problem Description

Inspired by the discussion in CTGAN #361, a user may accidentally save their synthesizer before fitting it. If this happens, then you will not be able to re-load and sample from the synthesizer, as the saved synthesizer has not been fit.

Expected behavior

If, for any synthesizer, the user calls save and the synthesizer has not yet been fit, show a warning:

Warning: You are saving a synthesizer that has not yet been fit. You will not be able 
to sample synthetic data without fitting. We recommend fitting the synthesizer first 
and then saving.

Additional context

Should this be an error instead? No because maybe there is a legitimate reason for saving it before fitting. For example, user has added constraints, updated transformers, etc. and wants to save the progress.