generic features like inv_sixth_ISI in Python API

[anilbey]

          It might also be a good idea to allow users to create/use from generic features like inv_sixth_ISI

or inv_6th_ISI, inv_seventh_ISI is the data has it. These can then also be used with BluePyOpt and BluePyEModel for optimisation and validation of e-models.

As you suggested these generic features can also be implemented in python for other features as well. Not necessarily on this PR.

Originally posted by @darshanmandge in #334 (comment)