Improve unit tests

[aburrell]

The following routines are not completely covered by the current unit tests and should be:

• [x] ocbpy/boundaries/dmsp_ssj_files.py
• [x] ocbpy/ocb_scaling.py
• [x] ocbpy/instruments/general.py
• [x] ocbpy/instruments/pysat_instrument.py
• [x] ocbpy/boundaries/files.py
• [x] ocbpy/tests/test_ocb_scaling.py
• [x] ocbpy/tests/test_boundary_dual.py