Implement AttrsDict.group(), syntactic sugar for AttrsDict.map(..., unique=False)

[gipert]

Slight syntax improvement:

>>> ids = (
...    lmeta.channelmap()
...    .group("location.string")[1]
...    .map("daq.rawid")
...).keys()
dict_keys([1104000, 1104001, 1104002, 1104003, 1104004, 1104005, 1105600, 1105602, 1105603])

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (de0da42) 72.72% compared to head (3ad061a) 72.60%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #48      +/-   ##
==========================================
- Coverage   72.72%   72.60%   -0.13%     
==========================================
  Files           7        7              
  Lines         737      741       +4     
==========================================
+ Hits          536      538       +2     
- Misses        201      203       +2     

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