GPIO ports are a shared resource

[sailedeer]

Since GPIO ports and pins are a shared resource (i.e. there are a discrete number of pins, and two references to a pin shouldn't exist at the same time). Need to include a small lookup table in memory for the port and the pin. Of course, the user can write directly to the memory-mapped register but there isn't much we can do about that.

In general, when writing a new MSP430 library it will probably be good practice to implement a similar "lock" mechanism.