Add support for reading filterbank files with nsamples parameter in header

[aaronpearlman]

nsamples is a valid keyword parameter in the header of filterbank files.

Currently, when reading filterbank files with this keyword, the following warning appears:

WARNING:root:Unknown header parameter: nsamples. Skipping it and continuing. This may lead to incorrect header values.
WARNING:root:Skipping next 4 bytes of data.

This also results in an incorrect number of samples being read from the file.

This can be fixed simply by adding the following to the SigprocFile class in your/formats/pysigproc.py:

_type["nsamples"] = "int"

[devanshkv]

Thanks, @aaronpearlman, would you send a PR for the fix?