BUG: raise NotImplementedError on passing of compound dtypes in constructors (GH5191)
Merge pull request #5192 from jreback/compound_dtypes

BUG: raise NotImplementedError on passing of compound dtypes in constructors (GH5191)
BUG: Bug in comparing duplicate frames (GH4421) related
BUG: Bug in describe on duplicate frames
Merge pull request #5194 from jreback/dup_eq

BUG: Bug in comparing duplicate frames (GH4421) related
CLN/ENH: Stop instantiating all offsets on load.

Use a more flexible lookup based on prefix then delegate creation to
class' `from_name()` method. Totally get rid of hasOffsetName (not necessary).
Finally, make some inheriting go on to simplify things.