Add vapour pressure deficit

[tlogan2000]

Vapour pressure deficit (VPD) is used by many forestry / biological partners as a measure of moisture stress. It is calculated via saturation vapour pressure and relative humidity (both of which are already available in the api).

Currently we can do a fairly simple manual calculation after calculating the two components but it should be simple to implement directly in the _conversion functions

VPD = (1 - (RH/100))*SVP
http://cronklab.wikidot.com/calculation-of-vapour-pressure-deficit

Contribution

• I would be willing/able to open a Pull Request to contribute this feature (but I'm not able).

Code of Conduct

• I agree to follow this project's Code of Conduct

[aulemahal]

Nice, even has a standard name : water_vapor_saturation_deficit_in_air.