load simple and multiple file types

[aburrell]

Addresses #25 and #21.

import datetime as dt
import pysatMadrigal
import pysat

stime = dt.datetime(2011, 8, 16)
etime = stime + dt.timedelta(days=2)

dmsp = pysat.Instrument(inst_module=pysatMadrigal.instruments.dmsp_ivm, tag='utd', inst_id='f15')
dmsp.download(start=stime, file_type='hdf5')
dmsp.download(start=stime+dt.timedelta(days=1), file_type='simple')
dmsp.load(date=stime, end_date=etime)
print(dmsp.data)

This should load 2 days of data, one of which is from an HDF5 file, the other of which is from a simple.gz file.

[rstoneback]

Thanks @aburrell!