Analysis functions for diatomic molecules.

[Alexsp32]

Changes:

• Added new DynamicsOutputs for Desorption Angles and DynamicsVariables during desorption (#317)
• Added distance, centre of mass, reduced mass function with and without check for closer periodic copies to a Structure submodule.
• Added an Analysis submodule to contain functions needed for more complex analysis of MD trajectories.

[Alexsp32]

Need to re-work several bits of this PR since they're very memory-inefficient and some edge cases aren't covered.

[Alexsp32]

DesorptionFrame - No views: 1.191380 seconds (7.33 M allocations: 486.175 MiB, 6.54% gc time, 84.50% compilation time: <1% of which was recompilation)
DesorptionFrame - With views: 0.401479 seconds (4.67 M allocations: 309.999 MiB, 6.78% gc time, 58.73% compilation time)
DesorptionAngle - No views: 0.645341 seconds (5.03 M allocations: 335.521 MiB, 4.93% gc time, 73.09% compilation time: 12% of which was recompilation)
DesorptionAngle - With views: 0.446124 seconds (4.71 M allocations: 313.256 MiB, 4.90% gc time, 61.16% compilation time)

I've merged in various changes to decrease the memory footprint of OutputDesorptionAngle and OutputDesorptionTrajectory and added a unit test for an example system, so I think this is now ready to be merged.

[reinimaurer1]

Nicely done!

[Alexsp32]

Docs preview here

[reinimaurer1]

@wgst there are a few new functions that could be useful for you