DataSet.hkl_to_asu(anomalous=True) should map centric reflections to Friedel-plus ASU

[JBGreisman]

When DataSet.hkl_to_asu() is called with the anomalous=True flag, reflections are mapped to the Friedel +/- ASU. This makes it useful to construct calls using DataSet.groupby(["H", "K", "L"]) that handle Friedel pairs separately. However, all reflections are only defined as Friedel +/- based on the M/ISYM flag (odd are Friedel+, even. are Friedel-), even if they are centric.

Example:

import reciprocalspaceship as rs

unmerged = rs.read_mtz("tests/data/algorithms/HEWL_unmerged.mtz")
unmerged.label_centrics(inplace=True)

example = unmerged.loc[[(11, 11, 8), (11, -11, -8), (-11, -11, -8)], ["BATCH", "CENTRIC"]]
print("Observations:")
print(example)

not_anom = example.hkl_to_asu(anomalous=False)
print("Friedel + ASU:")
print(not_anom)

anom = example.hkl_to_asu(anomalous=True)
print("Friedel +/- ASU:")
print(anom)

Outputs:

Observations:
            BATCH  CENTRIC
H   K   L                 
 11  11  8    454     True
    -11 -8    909     True
        -8    474     True
-11 -11 -8    203     True
        -8    814     True
        -8    627     True
Friedel + ASU:
         BATCH  CENTRIC  M/ISYM
H  K  L                        
11 11 8    454     True       1
      8    909     True       4
      8    474     True       4
      8    203     True       2
      8    814     True       2
      8    627     True       2
Friedel +/- ASU:
            BATCH  CENTRIC  M/ISYM
H   K   L                         
 11  11  8    454     True       1
-11 -11 -8    909     True       4
        -8    474     True       4
        -8    203     True       2
        -8    814     True       2
        -8    627     True       2

This behavior should be modified to only be used for acentric reflections -- centric reflections should not be considered "Friedel", and should only be mapped to the Friedel-plus ASU. The above example should give identical results for hkl_to_asu() with anomalous=True and anomalous=False.