new grid class for `OmnigenousField` coordinates

[ddudt]

The OmnigenousField class works in a $(\rho,\eta,\alpha)$ coordinate system. In the present code, it assumes a grid in $(\rho,\theta,\zeta)$ flux coordinates and maps them accordingly, but this is a hack. Once #2053 is completed, we should create a new grid class to represent this omnegenity coordinate system.

[ddudt]

May also want a generic 2D grid for use with Surface classes.