ENH: Improve par-rec support

Goals:

* [x] Detect and correct mis-ordered acquisition (e.g., x, y, t, z).
* [x] Detect and return multiple echoes in dataset.
* [ ] Detect and optionally permit missing data (e.g., run too short, doesn't contain full dataset). Current support is partial, since differing scale-factors will not work with truncated recordings currently.
* [x] Add file overwrite checks and option to overwrite.
* [x] Correct output if there are multiple data types in same recording with different scale factors (phase contrast vs complex).
* [x] Output DTI b-values and vectors, if recorded.
* [x] Output dwell time.

Although it's not 100% complete, it would be good to get a round of review to make sure I'm not way off.
Right - yes - the units thing would go very well in
PARRECHeader.as_analyze_map  - thanks again for working on this.  I'll help
as much as I can...
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