External dense SWARM error matrix being loaded even if dta != 4 months

Currently, we can specify pygeodyn to use the SWARM error matrix stored as R_dense_swarm.txt inside data/observations/GOVO/SWARM_err/

However, it is calibrated only every-4-months observations and should be used only with an analysis timestep of 4 months. However, the matrix is still loaded even if timestep != 4 months, as long as we specify SW_err str dense in the configuration file.

A possibility would be to flag out when we are trying to use this dense matrix with inappropriate timestep, and automatically switch to a diagonal matrix.