Add Stokes I only submode to idg-lib

IDG always assumed that both the input and output have 4 correlations/polarizations (XX, XY, YX, and YY), even when only XX and YY are really needed (to construct Stokes I). The code is now modified to also support only two correlations as input (XX and YY) and to produce only a single Stokes I grid. This saves many unneeded computations and reduces the memory footprint considerably. The a-terms are still 2x2 matrices, of which only the diagonal entries are really used in the new mode.

For now, this mode is only available in the example and test code in idg-bin, by setting STOKES_I_ONLY=1 in the environment.