Add preprocessed grids for certain fetch calls

[mdtanker]

Description of the desired feature:

Currently, all the fetch calls automatically return the full resolution of the grids. For example, DeepBedMap's initial spacing is 250m. Any spacing supplied by the user results in a pygmt.grdfilter and grdsample, which takes a long time for these large datasets. We should include low-resolution versions of each of these which are automatically returned unless the user specifies a higher res.

For now, these datasets include ice_vel, bedmachine, bedmap2, deepbedmap.

See the low and high res preprocessing versions of fetch.ice_vel() for how to implement this.

I think 5km would be a good spacing for the low res versions. Any added datasets with original grid spacing smaller than 5km should include a low-res version.

Are you willing to help implement and maintain this feature?

Yes

[mdtanker]

reconfigured the ice_vel call in PR #101.

Not sure how this should work with bedmap2 or bedmachine since they have many combinations of different layers and reference frames. Should we proprocess all grids at multiple resolution for both the geoid and the ellipsoid? Will be convenient for quickly working with the data, but at the cost of large amount of disk space.