3. Check for polarisation info in the sky model

[JLBLine]

While reading in things, register whether we need to calculate QU and/or V. Currently we have a do_QUV switch inside the GPU code, so will need to revisit that and see if it's enough. But at the very least, set do_QUV=1 in woden_settings if we have any polarisation info in the sky model.

IF we want to be efficient, we should really only be setting do_QUV for visibilities that have full pol as apply_beam_gains_stokesIQUV is far more expensive than apply_beam_gains_stokesI. But it'll take a decent bit of work to get that indexing correct

Testing need to update cmake_testing/wodenpy/skymodel/test_read_FITS_skymodel_chunk.py to check all these news values are read in, and update cmake_testing/wodenpy/skymodel/common_skymodel_test.py and cmake_testing/wodenpy/skymodel/read_skymodel_common.py with new polarisation options. Should probably write a new test test_read_yaml_radec_count_components.py as we don't explicitly test that at the moment (it's tested within test_read_FITS_skymodel_chunk.py)

[JLBLine]

I think it makes sense actually to have do_QUV set inside each components_t; that way we have some kind of granularity around doing full pol vs doing just stokes I, depending on what is inside that particular chunk of the sky model

[JLBLine]

Really should be setting up some tests that check for incorrect sky model formatting, e.g. missing V_MOD_TYPE and LIN_MOD_TYPE columns. At a minimum, should have some warnings spat out if multiple model types are present and we end up assuming a type.

[JLBLine]

If we're being REALLY smart, we should come up with a way to not calculate Stokes I when a source doesn't have Stokes I (e.g. it's just doing a linear polarised power law or something like that). It's a whole kernel call we can avoid. BUT need to think hard about how that source would get chunk, as things get ordered by the Stokes I flux model type. Might have to just assign Stokes I 'pl' to them all, chunk 'em as normal, then adjust the "num_power" numbers after the fact? Needs more thought that

[JLBLine]

Done in 061260f