Add `Var.convert_units!`

The non in-place version already exists as `Var.convert_units`. It would be good to add an in-place version to avoid unnecessary allocations.

Start by locating Var.convert_units and tracing how the existing non-in-place conversion handles its arguments and unit changes. Add the in-place Var.convert_units! counterpart so callers can convert without unnecessary allocations, then verify that it mutates the expected Var and preserves the behavior of Var.convert_units.

Written by the indexing model from the issue text.