Select points with real world coordinates, rather than grid coordinates

[jfrost-mo]

What problem does your feature request solve?

Currently CSET users must input coordinates relative to the grid of the data, which is often rotated for regional use cases.

Describe the solution you'd like

We should make it so inputted coordinates are treated as real world coordinates (probably on WGS84), and then transform them to the model grid.

Describe alternatives you've considered

Would there be any cases where it would be useful to input the model grid coordinates directly?