fix: don't copy unit registry if it is already the same thing

[Xarthisius]

I'll admit: I don't know what I'm doing. If this gets merged in, there is a chance some unnamed PhD student will probably lose a couple of weeks trying to debug a subtle issue of nonconforming units somewhere affecting their results. They'll git bisect and then git blame and learn my name. They won't know me, but they'll hate me. With passion.
However, on a slim chance that what I'm suggesting doesn't break the Universe horribly, let's run a full testsuite on this PR. Afterwards, I'll provide a concrete example how this benefits a specific use case.

[matthewturk]

[Xarthisius]

It turns out this change doesn't do much. It's one of those cases where you cannot trust a profiler (whether it's pyinstrument or line_profiler)

(main) $ kernprof -lv doc/source/cookbook/hse_field.py
Total time: 43.1662 s
File: /home/xarth/codes/xarthisius/yt/yt/data_objects/construction_data_containers.py
Function: _sanitize_edge at line 754
Line #      Hits         Time  Per Hit   % Time  Line Contents
==============================================================
   754                                               @profile
   755                                               def _sanitize_edge(self, edge):
   756      1182       3335.0      2.8      0.0          if not is_sequence(edge):
   757                                                       edge = [edge] * len(self.ds.domain_left_edge)
   758      1182      31009.0     26.2      0.1          if len(edge) != len(self.ds.domain_left_edge):
   759                                                       raise RuntimeError(
   760                                                           "Length of edges must match the dimensionality of the " "dataset"
   761                                                       )
   762      1182       1004.0      0.8      0.0          if hasattr(edge, "units"):
   763      1182   43113203.0  36474.8     99.9              edge_units = edge.units.copy()
   764      1182       4201.0      3.6      0.0              edge_units.registry = self.ds.unit_registry
   765                                                   else:
   766                                                       edge_units = "code_length"
   767      1182      13411.0     11.3      0.0          return self.ds.arr(edge, edge_units, dtype="float64")

(this PR) $ kernprof -lv doc/source/cookbook/hse_field.py
Total time: 0.034908 s
File: /home/xarth/codes/xarthisius/yt/yt/data_objects/construction_data_containers.py
Function: _sanitize_edge at line 754

Line #      Hits         Time  Per Hit   % Time  Line Contents
==============================================================
   754                                               @profile
   755                                               def _sanitize_edge(self, edge):
   756      1182       3013.0      2.5      8.6          if not is_sequence(edge):
   757                                                       edge = [edge] * len(self.ds.domain_left_edge)
   758      1182      29481.0     24.9     84.5          if len(edge) != len(self.ds.domain_left_edge):
   759                                                       raise RuntimeError(
   760                                                           "Length of edges must match the dimensionality of the " "dataset"
   761                                                       )
   762      1182       1028.0      0.9      2.9          if hasattr(edge, "units"):
   763      1182        959.0      0.8      2.7              if edge.units.registry is self.ds.unit_registry:
   764      1182        427.0      0.4      1.2                  return edge
   765                                                       edge_units = edge.units.copy()
   766                                                       edge_units.registry = self.ds.unit_registry
   767                                                   else:
   768                                                       edge_units = "code_length"
   769                                                   return self.ds.arr(edge, edge_units, dtype="float64")

I really wish I could say I shaved 40s off, but:

(master) $ time python doc/source/cookbook/hse_field.py
real	0m24.542s
user	0m24.367s
sys	0m0.176s

(this PR) $ time python doc/source/cookbook/hse_field.py
real	0m24.794s
user	0m24.782s
sys	0m0.955s

While I still believe this change is "the right thing to do"^{TM} it's suddenly way less exciting...

[neutrinoceros]

@Xarthisius do you still think this is worth merging ?

[matthewturk]

I think this is still worth merging.

[neutrinoceros]

sounds like we're at two implicit approvals. I'll formalise mine