ENH: modernize code in dimensionSet, dimensionSets

- 'unfriend' operators on dimensionSet, since they operate without
  requiring access to non-public members.

- add missing invTransform() function for dimensionSet.

- make inv(const dimensionSet&) available as
  operator~(const dimensionSet&), which can be used instead
  of (dimless/ds).

applications/test/dimensionSet/Make/files

@@ -0,0 +1,3 @@
+Test-dimensionSet.C
+
+EXE = $(FOAM_USER_APPBIN)/Test-dimensionSet

applications/test/dimensionSet/Make/options

@@ -0,0 +1,2 @@
+/* EXE_INC = */
+/* EXE_LIBS = */