ENH: restructuring of ensight output files

- changed ensightOutput from a class solely comprising static methods to
  a namespace and added in sub-namespaces Detail and Serial.

  This makes it easier to "mix-in" functions at different levels.
  Refactored and combined some serial/parallel code where possible.

  The general ensightOutput namespace has now shifted to be in the
  fileFormats lib, while leaving volField outputs in the conversion lib
  and cloud outputs in the lagrangian-intermediate lib.

  The ensightCloud namespace is now simply folded into the new
  ensightOutput namespace.

  These changes clean up some code, reduce fragmentation and
  duplication and removes the previous libconversion dependency for
  sampling.

- use int for ensight nTypes constexpr

Note: issue #1176 is unaffected except for the change in file name:

   ensightOutputTemplates.C -> ensightOutputVolFieldTemplates.C