Multiple fixes corresponding to Pulseq commits (see description)
- Updated calc_rf_center.py to make it compatible with a wider variety of pulses (https://github.com/pulseq/pulseq/commit/71aa176ca4c64660e1a5ec6780d57e1ad2502826)
- Updated check_timing.py to catch the negative delay error (https://github.com/pulseq/pulseq/commit/423575373571a1fb43a1eb1fadee76c2d019ffd6)
- Fix bug in split_gradient_at.py appearing when using it for the second time or on shaped gradients (https://github.com/pulseq/pulseq/commit/3587aa80ce2080bd683e53c78d9fe184676b5b73)
- Fixed pulse flip angle normalization for complex (non-real) arbitrary shape pulses (https://github.com/pulseq/pulseq/commit/bc5ec8482989aec8fa517ca275a184f1c177ff17)
- Improved first/last calculation when splitting shaped gradients (https://github.com/pulseq/pulseq/commit/47d996af3c50bd85f7a6e69067be09a0ade7b11f)