Fixed the calculation to use airmass, Alt, Az calculted by the sky

brightness model

- Changed the way airmass was used, allowing a way of providing the
  value (only for calculating transmission)
- Checked that tests pas as expected
- Also fixed up the demo file to explain things a little better
- Few more docstrings

	modified:   examples/Demo_SkyBrightness.ipynb
	modified:   obscond/skybrightness.py
	modified:   tests/test_skybrightness.py

Author: rbiswas4