MetaData rework

[qubixes]

This PR improves the internals of the MetaData class. Instead of relying on the PRC iRODSMeta class, a new class is created (MetaDataItem) which encapsulates one entry of the metadata for a data object or collection.

New features:

• Use meta["some_key"] to quickly get an entry for some key.
• Use meta.find_all(key=..., value=..., units=...) to find metadata entries.
• Rename keys/values/units with meta["some_key"].key = "new_key"

Fixes #277 (Open a new issue in the GUI repository if necessary)

[chStaiger]

Some suggestions for error messaging:

• If the key is set to something else as string or None, the error message is not very user friendly:

  meta["test"].key = meta
  TypeError: expected string or bytes-like object, got 'NoneType'
  ...
  

• When the key or value is set to '' --> Bad_AVU_Field: AVU attribute ('') is zero-length.

I think we should catch them and turn them into something like ValueError('Metadata key must be string and not empty.') and for the value respectively.

[chStaiger]

A very nice new feature and reorganisation of the code!!! Many thanks!
The only thing that needs to be adjusted are the exception strings mentioned above. They might not be understandable by an iRODS rooky.

[chStaiger]

A very useful function!!!

[JTvD]

This PR has quite some feature that are going to make life a lot easier, great work!