mean error and root mean square error profiles

[mo-tomosevans]

What problem does your feature request solve?

Adds mean error and root mean square error profiles for diagnostics, emulating the profiles available in the UK Trialling version of the RES.

Describe the solution you'd like

A new profile recipe using an operator calculating model vs obs. #403 needs to be done (at least for some obs sources) as a prerequisite.

Describe alternatives you've considered

[mo-tomosevans]

These diagnostics are calculated using VER for the RES.
It looks like it is as simple as it seems:

$\mathrm{ME} = \frac{1}{N}\sum_{i=1}^{N}(f_i - o_i)$

$\mathrm{RMSE} = \sqrt{\frac{1}{N}\sum_{i=1}^{N}(f_i - o_i)^2}$

Having spoken with Rachel, this could be done through MetPlus.