- added SAMD21G17D (tested) and 17L (untested) header files.
- adjusted sam.h, samd.h, and samd21.h to include the above.
- added ptc.h files to include (component and instance)
- added tcc3.h to include instance
- kept backwards-compatible with NMI and SVC as main handler calls, added #defines for NonMaskInterrupt and SVCall for new 17D and 17L terminology.