Prepare for pending deprecation of .drop() from xarray, use drop_vars or others instead.

[brews]

Automated tests trigger loads of warnings about this. For e.g.

tests/test_input_damages.py::test_compute_ag_damages
  /Users/brewstermalevich/Projects/dscim/src/dscim/preprocessing/input_damages.py:426: PendingDeprecationWarning: dropping variables using `drop` will be deprecated; using drop_vars is encouraged.
    .drop("variable")

Should move the code away from using .drop() on xarray structures before this starts to throw errors.