traj_to_gif updates

[robinsonmt1]

• [x] Automate povray generation: Already automated functionality in ASE. From first look, write('atoms.pov', ...) from ase.io produces atoms.pov and atoms.ini files; png files are produced using

  renderer = write('atoms.pov', ...)
  renderer.render()

• [x] Add optional flags for manual ase_gui rendering or automated povray rendering
• [x] Add an optional flag for whether you want to view the ase_gui
• [x] Change os command to prevent adding additional dependencies. Akash suggests matplotlib has .png -> .gif functionality, Pillow has .png -> .gif functionality and is already a dependency for ASE

[codecov[bot]]

Codecov Report

Attention: 14 lines in your changes are missing coverage. Please review.

Comparison is base (6616f08) 87.50% compared to head (ab4b10c) 87.80%.

Files	Patch %	Lines
carmm/analyse/traj_to_gif.py	79.36%	13 Missing :warning:
carmm/analyse/povray_render.py	97.82%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #114 +/- ## ========================================== + Coverage 87.50% 87.80% +0.30% ========================================== Files 74 76 +2 Lines 3033 3133 +100 ========================================== + Hits 2654 2751 +97 - Misses 379 382 +3 ``` | [Flag](https://app.codecov.io/gh/logsdail/carmm/pull/114/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andrew+Logsdail) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/logsdail/carmm/pull/114/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andrew+Logsdail) | `87.80% <91.41%> (+0.30%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Andrew+Logsdail#carryforward-flags-in-the-pull-request-comment) to find out more.

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