Non-const access to the internal field now obtained from a specifically
named access function consistent with the new names for non-canst access
to the boundary field boundaryFieldRef() and dimensioned internal field
dimensionedInternalFieldRef().

See also commit a4e2afa4

Avoids the clutter and maintenance effort associated with providing the
function signature string.

Also changed internalDegreesOfFreedom to be an integer type