ENH: allow "<case>", "<system>" ... in the string expansions (issue #792)

- the expansions were previously required as slash to follow, but
  now either are possible.

    "<case>", "<case>/" both yield the same as "$FOAM_CASE" and
    will not have a trailing slash in the result. The expansion of
    "$FOAM_CASE/" will however have a trailing slash.

- adjust additional files using these expansions