Add GaN_Systems_GaNPX-4_7x8.4mm

Add GaN-Systems GaNPX-4 7x8.4mm

Datasheet:

MR of the corresponding symbol:

I created a new footprint library (Transistor_Power.pretty) for this. Not sure, if this the right way. However, I haven't found any existing library which fits the footprint, since the footprint is special.

Further the pad shape is special. I left out the chamfer for pin 1 (right top of the pad), since it is not dimensioned in the datasheet.

I am also not sure about the naming of the footprint. The given footprint is unique for the GS66505B GaN-HEMT. However, on the website of GaN-Systems , there are also parts without an unique footprint.

Therefore, I am looking forward to your suggestions. KLC-check passed, when corresponding 3D-directory is added.