- 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  has since then been superseded by getOrDefault() with an optional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

- 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 this class to enumerate
  other things since it internally uses an 'int' for its values.

  Changed volumeType accordingly to enumerate on its type (enum),
  not the class itself.

- propagates check for lookupOrDefault etc.

- 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.

- 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.