Capitalize plane keys in Tfs class

[lmalina]

To improve consistency among our packages capitalize the plane keys for "planed" data frames, i.e. ("X", "Y") instead of ("x", "y").
Requires change of _define_property_two_planes(args, kwargs).

[JoschD]

Not in the file, names, right?

@lmalina

[fsoubelet]

Small write-up here so that it's always clear:

When a TfsCollection includes two planes, the attributes are appended with the plane keys (so we get, say, BET_x).

Previously the collection module implements this with a lowercase plane key (as in the example above). This issue is about respecting the pylhc convention of having this key capitalized (above example becomes BET_X).

The chosen implementation (see commit 37844ba later on corrected in dfbc675) uses .lower() in the __getitem__ and __setitem__ methods of the _TwoPlanes class. This way, internally the attributes are stored with a lowercase key, but when accessing them both lowercase and capitalized keys are accepted.

This is behavior is now also tested in test_collection.py.

Not in the file, names, right?

This was my mistake.
Convention for the file names across pylhc is still to have the plane key as lowercase.

@JoschD @lmalina please confirm this is correct.