incorrect handling of installation prefix

Thankfully as noticed in issue 1782 by @snaka, we have the same dash-bug 1757 here.

Somehow got missed in the submodule hierarchy. But also noticed that it currently defaults to installing in the project (OpenFOAM) directory, but like the other modules it should default to installing into the user directory and the -prefix option should be used to override.

@sbna - you probably haven't seen this in general (triggered largely as an older dash bug), but it may cause some headaches with your visualization installation (or not).