Deprecation warnings + bugfix

[mikeknep]

Deprecation warnings

Adds deprecation warnings guiding the user away from:

mt = MultiTable(rel_data, gretel_model="amplify")
mt.train_synthetics()

...and towards:

mt = MultiTable(rel_data)
mt.train_synthetics(config="synthetics/amplify")

Each line in the first block above will print a deprecation warning (but will continue to work). In a future release we'll remove the gretel_model parameter completely and make config a required argument to train_synthetics.

Bugfix

In the previous PR adding custom synthetics config support, I reused the existing supported_models property of the strategies for config validation. However, those values are essentially constants specific to Relational Trainer (the gretel_model param specifically); in the cases of amplify and actgan, they overlap with the model key we expect to see in a config, but that is not the case for:

gretel_model	model key
lstm	synthetics
tabular-dp	tabular_dp

The fix here is a bit verbose but only temporarily—we'll remove the supported_gretel_models property when we remove the gretel_model param.