What can be better?

Right now, these functions assume that pressure is in descending order, but there is nowhere users can find this information. I am not aware of other functions that have this issue, but here are a few things that can be done:

1. Provide a similar error message with CCL as EL and LFC do when pressure is not in descending error, i.e., metpy.calc.exceptions.InvalidSoundingError;
2. Specify that pressure should be in descending order in the docstring;
3. Manually check if flipping the pressure variable (and all other associated variables) will make it in descending order.