Add length support and cells property to MOC class

Implemented __len__ to return the number of orders at which
the MOC has cells, allowing len() to be used on MOC objects.

The cells property gives the total number of cells (at any order).
Add read and write methods to MOC class

These lazily import the required I/O methods from the
pymoc.io sub-package.

When reading from a FITS file, a new "include_meta" option is
turned on to recover metadata from the MOC table HDU's header.
A new test script has been added to test this functionality.
doc: Split the I/O functions into a separate page
Raise IndexError for invalid order in __getitem__

Changed this from ValueError to the more appropriate IndexError.