Add units check for to_pressure_coordinates

When converting to pressure coordinates without specifying a target pressure, the mapping is done by Plvl(z) = P0 * exp(-z / H_EARTH), where H_EARTH = 7000.0 and P0 = 1e5. The units should be checked, so that the conversion makes sense. For example, if it is kPa instead of Pa.

Locate the implementation of to_pressure_coordinates and inspect how pressure units are represented before the no-target-pressure mapping is applied. Determine the expected behavior for inputs expressed in units such as kPa rather than Pa, then verify that the conversion rejects or handles inconsistent units as intended.

Written by the indexing model from the issue text.