Re-factored the L2 projection; added all derham spaces.

Solves the following issue(s):

Closes #212 (closed), closes #240 (closed)

Core changes:

1. Moved geometric- and L2-projectors into a single file projectors.py; deleted geom_projectors.py. All projectors have the three methods get_dofs(), solve() and __call__().

   For the L2 projector, get_dofs replaces old method WeightedMassOperator.assemble_vec.

   L2 projector tests include: tests for all available mappings with callable and array input; a convergence test for singleton dimensions.

2. Added geom.-projection and L2-integration grid infos as attributes to the Derham class. pts, wts for both quad and projector grids; spans and bases for quad grids only.

3. Fixed tutorials, make html is working again (this had to do with squeeze_out from the last MR).

Model-specific changes:

None

Documentation changes:

An auto-doc section on the projectors was added.