Move coil meshing to MagnetCoil class

[gonuke]

The cubit calls to create a coil mesh should be a method of the MagnetCoil class. The MagnetSet class could then have a mesh generation method that calls the mesh gen on each of the coils.

[connoramoreno]

We might reconsider this issue in the context of the modified magnet modeling workflow. Meshing takes place after full coil construction, since the STEP file must be generated via CadQuery first, before being imported into Cubit. Furthermore, there's interest in providing an open-source volumetric meshing workflow (see #140).