amendments to quantity-kinds

Here's the changes for <common/c-15> / <common/quantity-kind> that we discussed earlier today.
1) Can you check I have used the correct dimensions for total precipitation (ML-2 ... I think this is kg/m^2) and total precipitation rate (ML-2T-1). 
2) I couldn't figure out what the dimension for pressure tendency, rate of change of pressure with respect to time, should be (currently left as "XXX"). Can you amend as appropriate.

Jeremy