Cleanup definitions.py

[paulmueller]

Once new data columns are computed in core.py (issue #8),

• remove tfd from definitions.py (move definitions to fmt_tdms.py)
• define a standard naming scheme (replace rdv and uid with something better).
• write everything in one list ([["area", "Area [µm²]"], ["areapix", "Area [px]"], ...])
• [ ] write dictionary class that obtains "Area [µm²]" with key "area" and "unknown" with key "unknown" to support new columns right away.

[paulmueller]

There are two problems with the last point:

1. Implementing the described classes will make the code less easy to comprehend.
2. The proposed changes will make dclab prone to typos (validity checks for column names will not be possible).

Even though being liberal regarding novel column names and allowing the user/developer to play around with new data, the conservative approach of hard-coding the column names in definitions.py is more secure and transparent at the moment.