guidelines.
* Uncommented assertions in operators and added check for operator[] index < 0.
* Moved one operator from private to public, this was a typo thing.
Now BusIn can also use PullUp, etc, instead of only BusInOut.
If the pin is NC, it does get to the init, but all write/mode functions
are disabled. This is how it used to be in the old gpio version. Quite
some libraries allow users to make pins NC, and they are all locking up
with the current mbed version. This is far from a perfect solution, but
more a temporary fix.