Replace GPy package with less messier alternative

[mcw92]

Currently, the Gaussian processes used in Propulate's surrogate models rely on the GPy package which repeatedly caused problems and errors, in particular in the context with matplotlib.
According to SheffieldML/GPy#1034, matplotlib is not listed in GPy's install_requires but necessary for some imports, causing errors.
That is why we manually install matplotlib in Propulate (and this is also the only reason why matplotlib appears in Propulate's dependencies). I find this very ugly.
Also, GPy is apparently incompatible with matplotlib v3.9.
We should try to get rid of GPy and find a better maintained alternative.
Also see SheffieldML/GPy#1029 😁.