Add support for circular FoVs elongated along RA

[jburba]

Add support for circular FoVs in the image domain model that are elongated along RA to account for the beam moving across the sky with time.

Currently, we only support modelling a single, circular pixel selection with a diameter equal to the specified model Field of View (FoV) at the central time in the input data. In reality, a drift scan instrument will observe a different patch of sky within the primary beam at each time. If we expand the HEALPix pixel filter to account for the beam moving across the sky, i.e. along RA, we can increase the accuracy of the image domain model while only increasing the complexity of the model along the $u$-axis of the model $uv$-plane.

New issue created in favor of #11 which has largely been addressed.