Pass all arguments by reference and update documentation.

All arguments for class constructions are now passed by reference.

The documentation, and in particular the formulae, have been updated.
Update documentation.
Invert arguments of cube constructors and sets the default value to 1.
Add value constructors and member access methods.
Correct typo
Add diffuse map unit tests.
Add ra() and dec() setting methods.
Update documentation.
Add ra() and dec() setting methods.
Update documentation.
Implement unit tests for radial and elliptical models.