Enhance dsamp to output SIGPROC (.fil) files as well as .h5 files

[Richard Elkins]

dsamp -h
usage: dsamp [-h] -s GROUP_SIZE in_path out_path

Downsample an input Filterbank file (.fil or .h5) to an output .h5 Filterbank
file.

positional arguments:
  in_path               Path of input Filterbank file (.fil or .h5)
  out_path              Path of output Filterbank file (.fil or .h5)

optional arguments:
  -h, --help            show this help message and exit
  -s GROUP_SIZE, --group_size GROUP_SIZE
                        Group size for the purpose of summing 2 or more time
                        samples. Required.