use dune-randomfield for computing the random parameter field

This resolves #7 (closed).

Tasks:

• add correlation lengths for each dimension via the stochastic.anisotropy key (value: axisparallel)
• revert 339e0de9 if merge request gets accepted
• use seeding functionality of dune-randomfield (see dune-randomfield#2]
• Add dune-randomfield to Docker image
• suggest three covariance functions: exponential, gaussian, spherical
• review tests: adapted dorie_muphi and dorie-rfg_pfg_0001

Open issues / questions:

• dune-randomfield does not support custom seeds (yet)