Add rescale filter and tests

[OpheliaMiralles]

Add Rescale / Convert filters. Rescale allows an operation of type scale*x+offset (and back). Convert takes unit_in, unit_out as arguments and performs a Rescale with adjusted offset and scale (and back).

Added tests using earthkit data mars "2t". Testing that the data min value is changed by the offset value, that the std is unchanged when scale=1, and multiplied by scale when it is different from 1. Test conversion from K to deg Celsius, Celsius to Fahreneit and back to K "manually" (using Rescale with the right constants). Then test conversion from K to deg Celsius using Convert.

Add cfunits dependency.

[FussyDuck]

All committers have signed the CLA.

[b8raoult]

Can you move the "import cfunits" in the method, so we only load it when needed.

[OpheliaMiralles]

Done