Update OpenFOAM version and use new package structures

As discussed via other channels, these are the goals of the changes for the openfoam package:

• packages that do not pollute /usr/bin, /usr/lib with hundreds of files
• support for activation, isolation of individual openfoam versions
• support a developer version (it is after all a toolkit)
• easier "launching" for the casual user
• finer package granularity for power users and/or sysadmins
• reduced patching

Some of these goals have been achieved within the debian packaging, others have been implemented upstream by modifying OpenFOAM to fit the requirements.

Prototype repos have been created for buster, bionic, eoan, focal under https://dl.openfoam.com with https://sourceforge.net/projects/openfoam/files/repos/deb for the backend. The debian/ definitions attached to this MR correspond to those repos (apart from minor version changes).

These repos can be referenced during testing as we iterate towards a suitable/agreeable debian package. It would also be conceivable to have the openfoam.com repos be regarded as previews for what would be available here.