Implement linearised harmonic term converters and add documentation

[fjclark]

Minimal functionality allowing the internal evaluation of linearised bond and angle terms was added in #130. This will be merged as-is to allow their use in fitting (in packages such as https://github.com/cole-group/presto/tree/devel) as soon as possible, but more functionality is required to:

• Allow the easy conversion from OpenFF Interchange objects to representations with linearised harmonics
• Allow the conversion to OpenMM systems from the smee representation

Also, documentation is needed.

This is being left to a later stage as it will require more thought, because:

• We need a way to select the linear angle/ bond converters over the already-present bond and angle converters
• The converter functions differ from all the others in that they require the user to select the basis

A rough suggestion on how this might work is given in #130 (comment):

• Create a function which replaces convert_bonds and convert_angles in the _CONVERTERS registry with convert_linear_bonds and convert_linear_angles functions with the basis specified in the function. Then convert_interchange could be used as normal. This could do something similar for the to OpenMM conversions at the same time.

Any comments and suggestions welcome! Thanks.