Add ensemble functionality

[KristianHMoller]

The underlying probabilistic advection method has the option of running an ensemble with noise. However, until now, this option has not been implemented into sunflow.

This PR allows for running as an ensemble by specifying the environment variable ENS_MEMBERS to be different from one. Per default, the output will be the ensemble median, but specifying --full_ensemble when calling sunflow will instead provide the full ensemble as output. The output_mode global attribute in the NetCDF file will specify the type of output: full ensemble, median or deterministic.

The default values of the noise parameters alpha and beta are 0 for deterministic mode and using other values will result in a warning.
For ensemble mode, currently, the default values are taken from Carpentieri et al.: https://www.sciencedirect.com/science/article/pii/S030626192301139X

Dependency: Requires this PR to be merged prior to yielding correct results: dmidk/SolarSTEPS#2

[KristianHMoller]

Scientific question 1: Which values to use for alpha and beta?
Currently using optimal values from Carpentieri et al., but these may be country-specific and are presumably derived with the potential error in the implementation (dmidk/SolarSTEPS#2).
Also: @irenelivia , we used slightly different values in our previous code. Do you recall why?

[KristianHMoller]

Implementation question 1: Do we want warning or failure if running deterministically (only one ensemble member) with noise?

[KristianHMoller]

Implementation question 2: Do we want the number of ensemble members and noise parameters to be CLI arguments rather than environemnt variables? I.e. parameters called when running sunflow: sunflow --ens_members 10?

[KristianHMoller]

Ideas for ensemble spread variables: Quantiles, 10/90 percentile

[KristianHMoller]

Mean vs. median - which is more useful?