Mrphy

- Swapped order of `{nT, xyz}`-axes in B-effective, and all other arrays
accordingly.
B-effective arrays of older versions that were saved to disk need to be
manually updated, i.e., swapping the two axes, before using in this version;
- Faster backward of `sim.blochsim`, at a cost of more VRAM usage;
- Fixed a numerical issue in `mrphy.utils.rf2lρθ` that might return NaN;

- Added sigmoid/logit based interior mapping for rf.
- Removed `cupy` from required dependencies, again.

- Added bare asterisk (`*`) argument to enforce usages of keyward arguments in some function calls.
- Replaced calls of `super()` to `object`.
- Made `SpinCube` a subclass of `SpinArray`, and removed redundant methods.
- Enable `copy.deepcopy` for `mrphy.mobjs`.

- 44% faster forward `sim.blochsim()` (on GPU)
- 47% faster backward `sim.blochsim()` (on GPU)
- Added `doUpdate` flag to `{SpinArray, SpinCube}.applypulse` to update
spin states after applying a pulse;
- Cleaned up and beautified codes and docs.
- `n_repeat` in `test_sims.py` for profiling

- Improved Exception and f-Strings usage;
- Updated documentation;
- `beffective` now able to handle `rf`, `b1Map` input that omitted the
`nCoil` dimension.
- version bumped from 0.1.6 to 0.1.8 due to a misoperation of pypi upload.