partial I/O for `UVCal`

[nkern]

It would be nice to have partial I/O capabilities for UVCal objects and calfits files. Just FYI, I did a line-profile on UVCal.read_calfits on a decently large file (~80 MB), and ~60% of the time was spent on line 434

self.gain_array = data[:, :, :, :, :, 0] + 1j * data[:, :, :, :, :, 1]

and 20% of the time was spent on lines 394-395

while 'HISTORY' in hdr.keys():
    hdr.remove('HISTORY')