- a wiki location seems to be more reasonable

Separate page for "running"

- collect information for shell session and openfoam-selector, which
  was previously scatttered in various places

Rename "page-build-code" to "building", with symlink for old location

Misc style changes

running/openfoam-selector.md

+<!-- --- title: openfoam-selector -->
+
+[![home](/icons/home.svg "wiki home")](/home)
+[![running](/icons/play.svg "running")](/running)
+
+The `openfoam-selector` package is used to register which
+openfoam packages are installed on the system and allows the user or
+system administrator select one of them.
+
+[[_TOC_]]
+
+Depending on your packaging system (or system administrator),
+`openfoam-selector` and `openfoam-selector-menu` commands may be
+available to define the preferred OpenFOAM version for the ***login***
+shell.
+
+To display which versions are installed:
+```
+$ openfoam-selector --list
+
+openfoam
+openfoam1812
+openfoam1906
+openfoam1912
+openfoam2006
+```
+To set a particular version
+```
+$ openfoam-selector --set openfoam2006
+```
+To query the current selection:
+```
+$ openfoam-selector --query
+```
+
+Most people will find the `openfoam-selector-menu` more convenient to
+use.
+```
+WARNING: Changes made to the openfoam-selector defaults
+will not be visible until you start a new shell!
+```
+
+***NOTE*** Similar to the behaviour of the `mpi-selector`, the values
+registered via the `openfoam-selector` will not be activated until the
+next full login. This is to avoid a possible clash of environments
+when a parallel job still being launched. Of course, you can still
+simply source the corresponding `bashrc` file directly. For example,
+```
+$ source /usr/lib/openfoam/openfoam2006/etc/bashrc
+```