ENH: add read guard for dimensionedType constructors (#762)

- deprecate dimensionedType constructors using an Istream in favour of
  versions accepting a keyword and a dictionary.

  Dictionary entries are almost the exclusive means of read
  constructing a dimensionedType. By construct from the dictionary
  entry instead of doing a lookup() first, we can detect possible
  input errors such as too many tokens as a result of a input syntax
  error.

  Constructing a dimensionedType from a dictionary entry now has
  two forms.

  1.  dimensionedType(key, dims, dict);

      This is the constructor that will normally be used.

      It accepts entries with optional leading names and/or
      dimensions. If the entry contains dimensions, they are
      verified against the expected dimensions and an IOError is
      raised if they do not correspond. On conclusion, checks the
      token stream for any trailing rubbish.

  2.  dimensionedType(key, dict);

      This constructor is used less frequently.

      Similar to the previous description, except that it is initially
      dimensionless. If entry contains dimensions, they are used
      without further verification. The constructor also includes a
      token stream check.

      This constructor is useful when the dimensions are entirely
      defined from the dictionary input, but also when handling
      transition code where the input dimensions are not obvious from
      the source.

      This constructor can also be handy when obtaining values from
      a dictionary without needing to worry about the input dimensions.
      For example,

         Info<< "rho: " << dimensionedScalar("rho", dict).value() << nl;

      This will accept a large range of inputs without hassle.

ENH: consistent handling of dimensionedType for inputs (#1083)

BUG: incorrect Omega dimensions (fixes #2084)