Change register addField method to have optional field bit interval mapping

As a user I usually prefer to assume a field is allocated in entirety to a register and that the bits are mapped in sequence.

eg. register[4:5] implicitly maps to field[0:1]

At present the method call is addField( fieldName, registerBitInterval, fieldBitInterval).

Change the call to have optional fieldBitInterval addField( fieldName, registerBitInterval, fieldBitInterval = None) and automatically calculate fieldBitInterval when the user does not provide it.