Adds a getAccel() member to BiorthBasis

[The9Cat]

Summary

This addresses the first of #136: a member function that returns the acceleration only, rather than the full field array with density and potential.

Details

• Added the getAccel(x, y, z) base class to BiorthBasis
• Implemented getAccel() for every derived basis: Spherical, Cylindrical, FlatDisk, CBDisk, Cube, and Slab
• getAccel() routines reimplement stripped down versions of the basis evaluations for speed
• Only Spherical (which includes SphericalSL by inheritance) and Cylindrical have been checked directly against their getFields() version.

To Do

Test the remaining acceleration routines:

• FlatDisk
• CBDisk
• Cube
• Slab

Note

This PR implements new features so is based off devel not main, as per our branch policy discussions.

[The9Cat]

Adrian and Lehman have checked this while developing their Gala interface. So I think this is safe to merge at this point.