Add documentation about uncertainty propagation

As commented on in #968 , it would be nice to have the math behind the uncertainty propagation in the analysis functions written out and documented. This might involve LaTeX to format things nicely.

Start by reading issue #968 and the analysis functions to identify the uncertainty-propagation mathematics that needs documenting. Use LaTeX where appropriate; done means the relevant derivation is written out clearly in the project's documentation.

Written by the indexing model from the issue text.