Reader cleaning

[ghiggi]

Prework

• I understand and agree to this repository's code of conduct.
• I understand and agree to this repository's contributing guidelines.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

The PR fulfills these requirements:

• It's submitted to the branch named as follows:
  • Add a reader: reader-<institute>-<campaign>
  • Fix a bug: bugfix-<some_key>-<word>
  • Improve the doc: doc-<some_key>-<word>
  • Add a new feature: feature-<some_key>-<word>
  • Refactor some code: refactor-<some_key>-<word>
  • Optimize some code: optimize-<some_key>-<word>

Summary

This PR cleans up most of the readers developed by @KimCandolfi.
It also introduces some further L0A processing checks:

• Coercion of possible numeric columns corrupted values to np.nan
• Strip trailing/leading space from string columns
• Ensure the data frame without duplicate timesteps (keeps the first)

In conclusion, this PR will help @regislon to set up the tests for most readers.
#130 lists some problems and TODO list related to remaining readers and the next improvements in L0A processing to be done.