Add units check for to_pressure_coordinates

[ph-kev]

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.

[ph-kev]

This is harder than I thought because you may write "Pa" or "kg m-1 s-2" for the units of pressure and both are equivalent.