Support multiple OpenFOAM versions with the same source

Often users are bound to specific OpenFOAM variants (e.g. openfoam.com, openfoam.org, foam-extend) and versions (not always the latest, sometimes very old). However, often small or not-so-small differences will give errors during compilation. Therefore, we need a system to be able to support different versions at compile time.

Implementing such a system would help a lot dealing with issues such as #21, #8, #9, #26. A list of currently supported versions can be found in our wiki.

Fortunately, all the three main variants set the environment variables WM_PROJECT (OpenFOAM for .com and .org, foam for foam-extend) and WM_PROJECT_VERSION (4.1, 5.0, 6 (no dot), ... and dev for .org, v1712, v1806, ... for .com, 3.2, 4.0, ... for foam-extend) (see also #29). Versioning systems and other assumptions have changed much lately.

We currently use the WMake build system, developped together with OpenFOAM. This is a quite primitive system, but it is intrinsic to OpenFOAM. On top of it, I have added the Allwmake bash script to automate the procedure even more (also a common way in OpenFOAM-derived projects).

A few ideas on how to go on:

1. (ugly) Support different versions in different branches. The current state of the branch works with the latest version, Git tags/other branches for older versions.

   • This is how we support multiple SU2 and CalculiX versions.
   • Difficult to maintain, but easier than other options (e.g. different directories)
   • Here we have multiple "latest versions".

2. (ugly) Use pre-processor commands to check the version and activate different parts of the code.

   • A bit ugly and maybe difficult to maintain
   • Some changes (e.g. required in #8 and #9 ) are much bigger and spread throughout the code to treat with small patches

3. (ugly) Have source files like Adapter_of6_dev.C and Adapter_of4_5.C and let our script pick the correct ones.

   • Difficult to maintain
   • Keep the files kind of clean

4. Use a proper configuration system (e.g. CMake)

   • Ok, but how to solve our problem with it?
   • Do we need to replace WMake completely or can it work on top of it?

I would very much value your feedback on this.