Add `measure_dp` and general tidying

[bwduncan]

Hi,

This patch adds a really simple method to calculate dew point, and tidies up the code a bit.

Thanks for a really helpful module :)

Bruce

[bwduncan]

Oh I forgot to say, I only tested this on a Wemos D1 mini v3.1.0 + SHT30 shield v2.1.0. I don't have any other hardware to test. Thanks!