- the Enum::readEntry() method was previously as bit sloppy with
  respect to the enumeration that it accepted. If the input was
  non-mandatory, typos would go unnoticed. Now tighten things so that
  if an enumeration is found, it must also be valid.

STYLE: remove unused/deprecated Enum::lookupOrFailsafe() method

- this was only used in a few places internally in 1712 and 1806 but
  has since then been superseded by getOrDefault() with an optional
  'failsafe' flag.

- can be used to check the validity of input values.

Example:

    dict.getCheck<label>("nIters", greaterOp1<label>(0));
    dict.getCheck<scalar>("relax", scalarMinMax::zero_one());

- use 'get' prefix for more regular dictionary methods.
  Eg, getOrDefault() as alternative to lookupOrDefault()

- additional ops for convenient construction of predicates

ENH: make dictionary writeOptionalEntries integer

- allow triggering of Fatal if default values are used

ENH: additional scalarRange static methods: ge0, gt0, zero_one

- use GREAT instead of VGREAT for internal placeholders

- additional MinMax static methods: gt, le

- this class really should not be used at all anymore.
  Add a message so people recognize can find any occurrences.

- Start brief descriptions with 'Deprecated(YYYY-MM)' so that it is
  readily visible in the short method description. Consistent date
  format (YYYY-MM), placed immediately after the \deprecated tag.

- more dictionary-like methods, enforce keyType::LITERAL for all
  lookups to avoid any spurious keyword matching.

- new readEntry, readIfPresent methods

- The get() method replaces the now deprecate lookup() method.

- Deprecate lookupOrFailsafe()
  Failsafe behaviour is now an optional parameter for lookupOrDefault,
  which makes it easier to tailor behaviour at runtime.

- output of the names is now always flatted without line-breaks.
  Thus,

     os << flatOutput(someEnumNames.names()) << nl;
     os << someEnumNames << nl;

  both generate the same output.

- Constructor now uses C-string (const char*) directly instead of
  Foam::word in its initializer_list.

- Remove special enum + initializer_list constructor form since
  it can create unbounded lookup indices.

- Removd old hasEnum, hasName forms that were provided during initial
  transition from NamedEnum.

- Added static_assert on Enum contents to restrict to enum or
  integral values.  Should not likely be using ...

- propagates check for lookupOrDefault etc.

- make the purpose more explicit, and reduces some work for the
  compiler as well.

- found() method for consistency with other classes

- operator()(name, deflt) for similarity to lookupOrDefault,
  but without a dictionary

- if the enum name is not found, then warn and return the default.

- eg, swak4Foam uses it for NumericAccumulationNamedEnum

- the NamedEnum wrapper is somewhate too rigid.
  * All enumerated values are contiguous, starting as zero.
  * The implicit one-to-one mapping precludes using it for aliases.
  * For example, perhaps we want to support alternative lookup names for an
    enumeration, or manage an enumeration lookup for a sub-range.

- Remove the unused enums() method since it delivers wholly unreliable
  results. It is not guaranteed to cover the full enumeration range,
  but only the listed names.

- Remove the unused strings() method.
  Duplicated functionality of the words(), but was never used.

- Change access of words() method from static to object.
  Better code isolation. Permits the constructor to take over
  as the single point of failure for bad input.

- Add values() method

- do not expose internal (HashTable) lookup since it makes it more
  difficult to enforce constness and the implementation detail should
  not be exposed. However leave toc() and sortedToc() for the interface.

STYLE: relocated NamedEnum under primitives (was containers)

- internal typedef as 'value_type' for some consistency with STL conventions