- syntax as per Bourne/Korn shell
      ${parameter:-defValue}

  If parameter is unset or null, the \c defValue is substituted.
  Otherwise, the value of parameter is substituted.

- eg,
       "$USER $(PWD) $USER"  ->  "username $(PWD) $USER"
  instead of
       "$USER $(PWD) $USER"  ->  "username $(PWD) username"

  this is noticable in some dynamicCode usages

STYLE: consolidate all string expand code into stringOps

Problems remain with codedFixedValueFvPatchScalarField:

- readIfModified() notices change on system/codeDict, but the
  codeProperties::setUnmodified() means that only a single entry will
  get processed

- it appears that while dlclose() may (or may not) be actually closing
  the library, there are probably still references about. This means
  that a subsequent reloading still points to the original functions
  and the lookup is not updated correctly.

- use newly introduced makeRemovablePatchTypeField to avoid this problem

- file changes were not getting noticed, made modified 'sticky'