BUG: fix value-entry behaviour in atmBoundaryLayerInlet (fixes #1578)

  DOC: improve atmBoundaryLayerInlet header documentation

  Without this change:
  - for serial-parallel computations, if `value` entry is available in
    an `atmBoundaryLayerInlet` BC, the theoretical ABL profile expressions
    are not computed, and the `value` entry content is used as a profile data
  - for parallel computations, if `value` entry is not available, `decomposePar`
    could not be executed.

  With this change:
  - assuming `value` entry is always be present, the use of `value` entry for
    the ABL profile specification is determined by a flag `initABL`
  - the default value of the optional flag `initABL` is `true`, but whenever
    `initABL=true` is executed, `initABL` is overwritten as `false` for the
    subsequent runs, so that `value` entry can be safely used.

  Thanks Per Jørgensen for the bug report.