Fixed `read_precognition()` as per #135 and updated tests

[dennisbrookner]

Accomplished all of the changes mentioned in #135 . Two outstanding thoughts:

• Though I did clarify in the docs that .ii files are allowed, I didn't want to change the name of the hklfile argument for fear of breaking backwards compatibility. That said, I know this function hasn't really seen a lot of daylight, so I'm happy to be overruled there.
• I added a ValueError for when a cell argument is provided but doesn't have length 6. Previously, the function checked for this length, but then just silently ignored an argument with the wrong length. I didn't add a test for this behavior, thinking that was overkill, but I could add that if desired.

Also, as best I can tell, this is the first "warning" in reciprocalspaceship. If some mechanism other than the built-in warnings module is preferable, let me know.

[codecov-commenter]

Codecov Report

Merging #136 (ea26024) into main (aca17c9) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #136   +/-   ##
=======================================
  Coverage   98.67%   98.67%           
=======================================
  Files          41       41           
  Lines        1583     1590    +7     
=======================================
+ Hits         1562     1569    +7     
  Misses         21       21           

Flag	Coverage Δ
unittests	98.67% <100.00%> (+<0.01%)	:arrow_up:

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Impacted Files	Coverage Δ
reciprocalspaceship/io/precognition.py	100.00% <100.00%> (ø)

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[kmdalton]

Jack will reply later, but imo type checks should live in the cellify decorator which is called by the setter.

[dennisbrookner]

Jack will reply later, but imo type checks should live in the cellify decorator which is called by the setter.

Yeah, Jack slacked me about this as well. Makes sense, I'll take that out.

[JBGreisman]

As mentioned above, the setter methods for DataSet.cell and DataSet.spacegroup implicitly handle all the type checking and coercion to gemmi objects. These use the @spacegroupify and @cellify decorators to ensure that all the relevant type-checking is handled in one place. This change was made in #94.

As such, this looks good -- when the DataSet attributes are set any necessary checks/conversion to gemmi objects will be done.