Add time lag calculation to SunPy Map

[wtbarnes]

The timelag method of Viall and Klimchuk (2012) is a useful technique for understanding large-scale cooling patterns across an entire AR. The algorithm can be implemented in pure Numpy provided the derotated and prepped images are arranged into a 3D array (with the 3rd axis being time).

Dask can also be used to parallelize the entire computation over many cores.

If this is appropriate to the scope of this package, I'd be happy to implement it.

[wtbarnes]

For anyone interested, you can find an existing Dask (and trivially numpy) implementation of this here.