Minor clean-up

  Within decomposeParDict, it is now possible to specify a different
  decomposition method, methods coefficients or number of subdomains
  for each region individually.

  The top-level numberOfSubdomains remains mandatory, since this
  specifies the number of domains for the entire simulation.
  The individual regions may use the same number or fewer domains.

  Any optional method coefficients can be specified in a general
  "coeffs" entry or a method-specific one, eg "metisCoeffs".

  For multiLevel, only the method-specific "multiLevelCoeffs" dictionary
  is used, and is also mandatory.

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ENH: shortcut specification for multiLevel.

  In addition to the longer dictionary form, it is also possible to
  use a shorter notation for multiLevel decomposition when the same
  decomposition method applies to each level.

Moved the snappyHexMesh tutorials to the appropriate solver directories and created links.