Alter field metadata

[sandorkertesz]

This PR adds the copy() method to fields. The result is a new field containing:

• reference to the values/values accessor in the original field
• a wrapped metadata object updated with the kwargs

The original field is kept unaltered.

E.g. if f is a field with "level" = 500:

>>> f_new = f.copy(level=700)
>>> f.metadata("level")
>>> f_new.metadata("level")
500
700

The kwargs can contain callables with the signature described below:

def _func(field, key, original_metadata):
    return original_metadata[key] + 100

f_new = f.copy(level=_func)
print(f.metadata("level"))
print(f_new.metadata("level"))

500
600

[codecov-commenter]

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[b8raoult]

original_metadata is a dict(), not a Metadata