BitField: Delete copy assignment to prevent obscure bugs.
Fix a few warnings caused by using BitField with non-typesafe functions.
BitField: Add an explicit getter function for retrieving the BitField value.

Sometimes (in particular when using non-typesafe functions) it can be convenient to have a getter method rather than performing a potentially lengthy explicit cast.
Merge pull request #483 from neobrain/bitfield_fixes

BitField fixes