Improve alignment of its behaviour with std::shared_ptr

  - element_type typedef
  - swap, reset methods

* additional reference access methods:

cref()
    returns a const reference, synonymous with operator().
    This provides a more verbose alternative to using the '()' operator
    when that is desired.

        Mnemonic: a const form of 'ref()'

constCast()
    returns a non-const reference, regardless if the underlying object
    itself is a managed pointer or a const object.
    This is similar to ref(), but more permissive.

        Mnemonic: const_cast<>

    Using the constCast() method greatly reduces the amount of typing
    and reading. And since the data type is already defined via the tmp
    template parameter, the type deduction is automatically known.

    Previously,

        const tmp<volScalarField>& tfld;

        const_cast<volScalarField&>(tfld()).rename("name");
        volScalarField& fld = const_cast<volScalarField&>(tfld());

    Now,

        tfld.constCast().rename("name");
        auto& fld = tfld.constCast();

--

BUG: attempts to move tmp value that may still be shared.

- old code simply checked isTmp() to decide if the contents could be
  transfered. However, this means that the content of a shared tmp
  would be removed, leaving other instances without content.

* movable() method checks that for a non-null temporary that is
  unique (not shared).