Mem error

Branch that that made pygeodyn less memory intensive when computing the PCA for the flows U, with the downside of being slower by a tiny amount (around 0.3 secs on a personal laptop). It simply delay the loading of the magnetic field and secular variation and wait for the PCA to be done. That way the space taken by magnetic data is not added on top of the PCA (which uses SVD, a linear operation that is memory costly) of the core flows.

If memory related problems arises again, more sophisticated solutions such as dask arrays could be used.