Basic conversion operators for humidity, temperature and pressure

[daflack]

Creates a series of basic conversion operators, not worrying about grid staggering or vertical levels at this point. The humidity, pressure, and temperature are all treated together due to the fact that they are strongly linked to each other and require the mix of code to be calculated.

Fixes #1852.

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• Documentation has been updated to reflect change.
• New code has tests, and affected old tests have been updated.
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[github-actions]

[daflack]

Conversions required, once basic code is done (excluding tests and full documentation will tick off)

A set of conversion operators for different humidity variables to be stored under `humidity.py:

• specific humidity to mixing ratio
• mixing ratio to specific humidity
• specific humidity to relative humidity
• mixing ratio to relative humidity
• relative humidity to specific humidity
• relative humidity to mixing ratio

A set of derived temperature operators in 'temperature.py':

• Virtual temperature
• Dewpoint temperature
• Potential temperature
• Equivalent Potential temperature
• Virtual potential temperature
• Saturation potential temperature

A set of derived pressure operators in 'pressure.py':

• Vapour pressure
• Vapour pressure from RH