TODOs for Monopole GR

[Yurlungur]

The following are TODOs for the MonopoleGR package after merging the current PR in:

• [x] Implement reduction of the fluid variables onto the 1D GR grid
• [ ] For Cartesian coordinates, cache metric gradients on the full 3D grid to avoid doing coordinate transformations
• [x] Test oscillating TOV star
• [ ] When interpolating, interpolate multiple quantities at once, reducing search cost time.
• [ ] Automatically or adaptively generate the MonopoleGR 1D grid.
• [x] Move TOV machinery into either pgen or a separate package.