Remove delr/delc from dataset

[dbrakenhoff]

I propose to remove delr and delc from the dataset entirely. Logic to determine delr/delc will be a separate function that can be called when necessary. Delr and delc are easy to compute from x and y, plus the extent.

The advantage of this change is that caching becomes cleaner (since we don't need to keep track of delr/delc if they are used in the computations for obtaining datasets). It also simplifies code because we don't have to check whether delr/delc are present in either the attributes or data variables.

• Fix all TODOS related to this, marked with TODO: * delr/delc *

[dbrakenhoff]

Addressed by #346