…em` type hints

… dead time

(This should be caught by `make_adc`. If we get here, the user really wants to adjust the delay and just accept it and let `check_timing` report it)

- Added unit test for `check_timing`
- The default signature of `seq.check_timing` remains the same, but can pretty-print the report with `print_errors=True`
- `seq.write` now checks sequence timing by default
- 'TotalDuration' calculation moved from `check_timing` to `seq.write`
- Check whether gradients in the last block ramp down to zero removed from `check_timing`

- Can be enabled/disabled with `pp.enable_trace(limit)` and `pp.disable_trace()`
- Added tracing to all blocks and all RF/ADC/gradient events
- Added printing of the trace to `check_timing` when it is available

…e `TotalDuration` in the sequence definitions

- `Sequence.write` now checks whether the last block ramps down to 0
- Fixed gradient continuity checks in `set_block`
  - Now handles arbitrary block orders properly
  - Now properly checks the last value of the previous block
  - Added gradient checks for blocks without gradients
  - Sequence object now remains valid after gradient errors
- Added unit tests for gradient continuity in `test_block`