ENH: minor cleanup of memory streams and IOstreamOption

- make memory streams header-only (simpler)
- add sub-views and direct seek for span streams

New IOobject convenience methods

- IOobject::instanceValue() : return the IOobject instance as a scalar
  value (or 0). Effectively the same as instant(io.instance()).value()
  but with far less typing.

- IOobject::fileModificationChecking_masterOnly() : combines checks for
  time-stamp and inotify variants

STYLE: minor adjustments for Enum

applications/test/IStringStream/Make/files

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