Rob/mdanse trajectory filter

[RobBuchananCompPhys]

Description of work
TODO:

(Probably don't need to solve all of these immediately - some may become issues for further work after this is completed)

UI

• Odd crash occurs when returning to TrajectoryFilter job page from another job, error appears to be due to deleted grid layout components in the trajectory filter widget
• Trajectory filter widget panel inputs move around alot in layout - will change to a grid layout for all input components to improve stability
• Allow user to visualise the attenuated trajectory power spectrum alongside filter (currently non-enabled "Show attenuation" checkbox)
• When x-axis is set to energy units, the axis contracts - this should remain the same scale as for frequency units

UI &/or Backend

• Number of simulation frames post-filtering exceeds original frame count
• Investigate cause of atoms collapsing to 'ugly blob' when certain filter variants (seems to be highpass and bandpass removing low frequency vibrations) are applied to trajectory
• Ensure the Z- and S- domain representations of the transfer function polynomial are correct

Backend

• Filter class and filter-variant child classes
• Write metadata (filter configuration)

Tests

• Basic unit tests for correct functioning of filter class
• Full end-to-end test for a more complex molecular dynamics trajectory

Other

• Ensure sensible default values for filters
• Complete numpy docstrings for methods
• Apply black formatting

Fixes
A list of fixes.

To test
Please describe the tests that were run to verify the changes.