support relative humidity and dewpoint temperature

[Zhangtc1013]

Hello, when will this library support relative humidity and dewpoint temperature

[fmaussion]

Thanks for your interest! I don't have time to add new variables but this is actually quite easy to do. See discussion here: #140

Submissions welcome! These two variables are very easy to add.

If you are in a hurry, wrf-python should have them already.