Downburst speed estimation

### What problem does your feature request solve?

The Microburst Windspeed Potential Index (MWPI; Pryor 2010, 2012, 2015) is a diagnostic aimed at identifying whether an environment could produce a downburst. This will be a really useful impact-based metric and also another identifier of the potential impact a model change has on convection. It can be used to then further calculate the Windspeed Gust Potenital (WGP) to convert into actual windspeeds in knots.

### Describe the solution you'd like

Operator for the MWPI and WGP and relevant recipes. Also required maybe a dewpoint temperature operator, and relevant diagnostics to help calculate the dewpoint temperature (saturation vapour pressure etc.)

For further details (including about reviewers) please refer to #1550.

### Required Diagnostics

Temperature at 850 hPa and (ideally 670 hPa, but 675 hPa works just as well)
dewpoint temperature at same levels (or means to calculate the dewpoint temperature)
Surface-based CAPE