Update rfg to randomfield v2.6

What does this MR do?

This is a follow-up to !151 (merged) which was intended to resolve #150 (closed). However, the dorie pfg module was not actually built or tested before and therefore incompatibilities slipped through.

• Update the dorie-rfg module/executable to dune-randomfield v2.6.
• Build dorie-rfg in build:debug and build:system-tests CI jobs.
• Add system tests which execute the dorie pfg module for several input test cases (no actual checks are performed).
• Update parfield Python module to use DUNE INI file parser instead of configparser which is incompatible to INI files with keys that do not belong to sections.

Is there something that needs to be double checked?

Not sure. The system tests are just execution tests—they simply check if the program runs with the proposed settings, but not if the result is correct.

Can this MR be accepted?

• Implemented updates to dorie-rfg executable and parfield module
• Added system test to check if dorie pfg can be executed
• Pipeline passing
• Added entry to CHANGELOG.md

Related issues

Closes #150 (closed)