provide additional text expansion shortcuts

In many places we use the string expansion to provide filenames. Eg,

"${FOAM_CASE}/comms"
"$FOAM_CASE/constant/reactions"
...

And then using the fileName::expand() method or stringOps::expand() to resolve them. Propose to provide some tag short cuts for less clutter. Eg,

"<case>/comms"
"<constant>/reactions"
...

The total number of tags is fairly limited in scope:

<etc>/        => user/group/other OpenFOAM directory
<case>/       => FOAM_CASE directory
<constant>/   => FOAM_CASE/constant directory
<system>/     => FOAM_CASE/system directory

We may consider alternative tag delimiters such as [case]/, but it should be easy to read/type and we should avoid {} delimiters since a simple typo has different meaning and would be a hard to trace input error (eg, ${case} vs {case}).