[ENH] BiphasicPulseTrain metadata

[jgranley]

Currently, manipulating BiphasicPulseTrains results in unexpected behavior when using with a BiphasicAxonMapModel. During creation, the amplitude, frequency, and pulse duration are stored in metadata, and are later used in the BiphasicAxonMapModel. If pulse train is modified in any way (e.g. multiplying by a scaler to scale amplitude), the metadata is unchanged, and the model will predict the same thing.

If this were updated, then ImageStimulus would be able to be used with amplitude encoding

Describe the solution you'd like
Modifying a BiphasicPulseTrain should either update the metadata if the resulting pulse is still a valid BiphasicPulseTrain, or it should somehow mark in the metadata that this is no longer a valid BiphasicPulseTrain so that the Biphasic model will reject the stimulus. The overloaded operaters will have to be overridden for BiphasicPulseTrain, and perhaps some metadata changes.