Volume cooling ellipsoids

[vaithak]

Adding a discussion here and closing this PR: #171.
The changes in PR have mostly been split into multiple PRs and have been merged, will revamp the remaining code in that PR and create a new standalone PR for the volume_cooling_ellipsoid method.

Further steps:

• rebase the code on the develop branch.
• replicate @TolisChal's matlab code - (this has been long due from my side 😅) .
• get the new code reviewed and take feedback.
  cc: @vissarion , @TolisChal