pyradiosky hdf5 format for FHD cubes

[jpober]

Find a way to interface pyradiosky's HEALPix tools / hdf5 format with FHD cube save files (i.e the inputs to eppsilon)

[bhazelton]

A couple notes on this:

1. This will require DFTing to be able to divide out the weights in UV space.
2. The cubes are in instrumental polarizations, which are not currently supported by pyradiosky.

[bhazelton]

Probably better to make the weights and variances arrays optional parameters on the object so they can all be written to the same file with the data array. The data array will need to have different units than normal since the sampling isn't divided out (it scales with the integration).