[ENH] MoleculeLoader support for PSeAAC

[prashantpandeygit]

resolves #46

Added MoleculeLoader as input type for PseAAC and AptaNetPseAAC, for the string input it is parsed as before (returns 1D array), and for MoleculeLoader (returns list of 1D arrays)

-tests to be added

[fkiraly]

Great! Thanks for contributing!

To align this with the current design: you should inherit AptaNetPSeAAC from BaseTransform.

Feel free to suggest minimal modifications to BaseTransform if it does not fit.

I would also move the PSeAAC algorithms to the trafos folder.

[prashantpandeygit]

for the BaseTransform, it plays with dataframe as input and output, so the _aptanet_utils.py needs to wrap each string into a dataframe, or should we still be accepting strings only? or better we could accept strings also and auto wrap them into a dataframe?

also tests for AptaNetPSeAAC needs to be changes to address dataframe .

[fkiraly]

BaseTransform should accept MoleculeLoader, that is how I would solve it