Version fusion

[MakisH]

This pull request isolates version-specific changes into separate files included at compile-time (we could even say "configuration-time"), trying to unify all the different variants of the adapter we are currently offering as branches. See #32 for a discussion of the problem (closes #32).

Motivation:

• Currently, with every version-specific branch we apply short patches.
• OpenFOAM already splits code into parts that it includes in the middle of the implementation using #include "file.H".

Idea:

• Put the parts that are version-specific into a separate directory variants/<variant>/version-specific/.
• Include them in the middle of the implementation, e.g. as #include "version-specific/file.H".
• Leave a comment with a concrete implementation example for readability (they must be short anyway).
• As <variant> we always take the $WM_PROJECT_VERSION. We only treat it as a string as it varies a lot among versions.
• We need a directory for every version. We define the files in the oldest supported version and create links for newer versions (to the complete directory and/or individual files).

Pros & Cons of this approach:

• (+) No need for the user to change branches, always have the latest changes in all variants
• (+) Easily extensible: Just find the closest version and copy the directory or make a link
  • Supporting a new version should usually mean "create a link to the previous version".
• (+) Avoids duplication: Only define the patch in one file, link from other versions
• (+/-) Clearly define which versions we support
• (-) For a new version we always need a new directory/link --> users should get the latest adapter or patch the change
• (-) To fix one version in a new location we need to update all the variants.
• (-) Reduced readability/refactoring capability
  • But OpenFOAM itself already uses such a preprocessor-based approach
  • and we keep a comment of the implementation
  • we will have an issue if fundamental functions we use all the time start changing (e.g. older .boundaryField() and .boundaryFieldRef().

Todo:

• Implement a warning/error for unrecognized versions
  • Can we assume and handle a "default" variant?
• Port changes from [Will not merge] Compatibility with OpenFOAM 6 #33 (OpenFOAM 6) (closes [Will not merge] Compatibility with OpenFOAM 6 #33)
• Port changes from [Will not merge] Compatibility with OpenFOAM 7 #91 (OpenFOAM 7) (closes [Will not merge] Compatibility with OpenFOAM 7 #91)
• Port changes from [Will not merge] Compatibility with OpenFOAM-dev #36 (OpenFOAM-dev) (closes [Will not merge] Compatibility with OpenFOAM-dev #36)
  • Open questions: are there issues with openfoam.org/dev and openfoam.com/dev?
• Implement changes from [Will not merge] Compatibility with OpenFOAM4 (4.0, 4.1) #120 (OpenFOAM 4) (closes [Will not merge] Compatibility with OpenFOAM4 (4.0, 4.1) #120)
• Check/update bundled tutorial scripts

Before merging, check that the final version builds with:

• OpenFOAM 4.1
• OpenFOAM 5.x
• OpenFOAM 6
• OpenFOAM 7
• OpenFOAM-dev
• OpenFOAM v1706
• OpenFOAM v1712
• OpenFOAM v1806
• OpenFOAM v1812
• OpenFOAM v1906
• OpenFOAM v1912
• OpenFOAM v2006

External updates:

• Check/update tutorial scripts (especially FSI tutorials)
• Update wiki page Notes on OpenFOAM
• Update system tests

Possible next steps:

• Automatically decide between pimpleFoam and pimpleDyMFoam in the tutorial scripts. Maybe simply apply a special case for the oldest supported versions.

[MakisH]

Since we decided (#52) to keep support for the Foundation branches and release archives for each range of supported versions, I am closing this for now. It is an idea that we may revive in the future, though, so I would like to keep the branch.