Adding few changes for metadata

[gviejo]

This PR fixed a few issues I found on metadata.

tsdframe = nap.TsdFrame(t=np.arange(10),d=np.random.randn(10,3), columns=['aa', 'bb', 'cc'])

This was working tsdframe[['aa', 'bb']]
This was not tsdframe['aa']

It would try to access ['a', 'a']

2. Changing
   elif isinstance(key, list) and all(isinstance(k, str) for k in key): line 348 of TsGroup to
   elif isinstance(key, list) and len(key) and all(isinstance(k, str) for k in key):

3. Improved repr of TsGroup. Was throwing an error when TsGroup was empty.

4. Changing the init of TsGroup within TsGroup to avoid deprecation message. **self._metadata[cols]) -> metadata=self._metadata[cols]

TODO :

Updating repr to remove vertical bars

To discuss to whether or not keep .loc in case you want to do column indexing with integers

1. .get_col method to replace .loc
2. adding a default column label "col_n" for TsdfFrame to remove ambiguity over numpy indexing.
3. Allow integers and maybe a warning.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
pynapple/core/interval_set.py	94.30% <100.00%> (+0.67%)	⬆️
pynapple/core/time_series.py	89.55% <100.00%> (+0.08%)	⬆️
pynapple/core/ts_group.py	92.34% <100.00%> (+0.07%)	⬆️
pynapple/io/interface_nwb.py	79.32% <100.00%> (ø)

... and 2 files with indirect coverage changes