Added fixed length args to TimeSeries.taper

Created by: tjma12

Adds duration and nsamples options to TimeSeries.taper() that allow user to specify a time duration or number of samples to taper rather than using the automatic tapering window. If neither are provided, the method defaults to the automatic tapering. If both duration and nsamples are provided, a ValueError is thrown to avoid mismatched taper arguments.

I also added a couple of explicit type casts to int that were causing issues when the specified number of taper samples was more than half of the duration of the data.