* make sigpy an optional dependency

* add code for the following adiabatic pulses:
bir4, hypsec, wurst, goia_wurst, bloch_siegert

* introduce sigpy mark for pytest

* add tests for `make_adiabatic_pulse`

* fix bug in padding code of `make_adiabatic_pulse`

* change some default parameters from 0 to None

* some internal variable renaming and refactoring


Co-authored-by: Patrick Schuenke <patrick.schuenke@gmail.com>