Coil-Coil & Plasma-Coil Minimum Distance Objectives

[dpanici]

• Adds the objective CoilsetMinDistance, which returns the minimum distance to another coil for each coil in a coilset. Resolves #898
• Adds the objective PlasmaCoilsetMinDistance, which returns the minimum distance to the plasma surface for each coil in a coilset. Resolves #900

Dependencies:

• #1016
• #1017
• #1018

TODO:

• [x] unit tests for CoilsetMinDistance
• [x] unit tests for PlasmaCoilsetMinDistance
• [x] regression test using both objectives in a coil optimization
• [ ] make it work with surface and/or equilibrium to resolve #947