Refactor for argmax and new method argmin

[sadielbartholomew]

We currently support an argmax method (see e.g. here) on the Data and Field classes, but not an equivalent argmin.

We've noted to add in argmin in time for the next minor release, but to do so by refactoring somewhat to create a common base for these methods (and potentially other similar ones).

[davidhassell]

Removing the 3.8.0 milestone, as the dask work (#182) will now take place first, and will affect the implementation of argmin/argmax. So this will be addressed at a version soon after 4.0.0 (i.e. the first dask version).