WIP: Improve regularisation documentation and add ADMT demo

[jacklovell]

• Add a Regularisation section to the tomography documentation, describing the functionality inside the admt_utils module.
• Fixup the docstrings in admt_utils.
• Add calculation of "skewed" second derivative operators, which operate along the diagonals of the grid.
• Add a bolometer diagonstic system to Generomak.
• Add a new example using the Generomak bolometers and the regularisation operators to perform isotropic and ADMT inversions.

Fixes #382