Get low res and high res pairs from different datasets (like ERA and WTK)

[bnb32]

Why this feature is necessary:
Training on ERA + WTK instead of coarsened WTK + WTK might improve results.

A possible solution is:
We will need to work out how to efficiently match low res and high res grids. This could be a pain in the case of ERA (a regular grid) and WTK (lambert conformal). Once the grid matching is worked out then matching high res and low res samples could be done with something like slice(high_res_start, high_res_end, t_enhance/s_enhance). Low res and high res datasets could be integrated into the batch handler by using separate data handlers for each and then refactoring the Batch class with the Batch.get_coarse_batch method. A regridding between the two data handlers could be precomputed.

[bnb32]

Completed with #162