Implement non-Hammurabi random fields

[luizfelippesr]

Currently, there are no proper IMAGINE random fields implemented.

Hammurabi has a few built-in options of random field, which IMAGINE can manipulate using Dummy fields, but that may not be enough: because Hammurabi is a Simulator, it has to run on every evaluation of the likelihood (to generate the observables), this means that the costly random field generation is re-done for each ensemble realisation at every evaluation of the likelihood.

If the random field is generated outside Hammurabi, by a dedicated Field class, there are several ways we can optimize our problem (perhaps allowing the use a large number of ensemble size). In particular: multiple realisations can be produced by adding a phase, instead of recomputing everything; parts of the calculation could be cached and reused during the sampling.

Thanks @amitseta90 for the discussion and suggestions!