Indexing last interval in an IntervalSet using -1 throws an error

[sjvenditto]

The following code snippet throws an error:

>>> ep = nap.IntervalSet(start=[0,2,4], end=[1,3,5])
>>> ep[-1]

The error trace:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[6], line 2
      1 ep = nap.IntervalSet(start=[0,2,4], end=[1,3,5])
----> 2 ep[-1]

File [/opt/homebrew/Caskroom/miniforge/base/envs/ccn-jan25/lib/python3.11/site-packages/pynapple/core/interval_set.py:438](http://localhost:8888/opt/homebrew/Caskroom/miniforge/base/envs/ccn-jan25/lib/python3.11/site-packages/pynapple/core/interval_set.py#line=437), in IntervalSet.__getitem__(self, key)
    435 elif isinstance(key, Number):
    436     # self[Number]
    437     output = self.values.__getitem__(key)
--> 438     return IntervalSet(start=output[0], end=output[1], metadata=metadata)
    439 elif isinstance(key, (slice, list, np.ndarray, pd.Series)):
    440     # self[array_like]
    441     output = self.values.__getitem__(key)

File [/opt/homebrew/Caskroom/miniforge/base/envs/ccn-jan25/lib/python3.11/site-packages/pynapple/core/interval_set.py:298](http://localhost:8888/opt/homebrew/Caskroom/miniforge/base/envs/ccn-jan25/lib/python3.11/site-packages/pynapple/core/interval_set.py#line=297), in IntervalSet.__init__(self, start, end, time_units, metadata)
    296 self._initialized = True
    297 if drop_meta is False:
--> 298     self.set_info(metadata)

File [/opt/homebrew/Caskroom/miniforge/base/envs/ccn-jan25/lib/python3.11/site-packages/pynapple/core/interval_set.py:1124](http://localhost:8888/opt/homebrew/Caskroom/miniforge/base/envs/ccn-jan25/lib/python3.11/site-packages/pynapple/core/interval_set.py#line=1123), in IntervalSet.set_info(self, metadata, **kwargs)
   1050 @add_meta_docstring("set_info")
   1051 def set_info(self, metadata=None, **kwargs):
   1052     """
   1053     Examples
   1054     --------
   (...)
   1122     shape: (3, 2), time unit: sec.
   1123     """
-> 1124     _MetadataMixin.set_info(self, metadata, **kwargs)

File [/opt/homebrew/Caskroom/miniforge/base/envs/ccn-jan25/lib/python3.11/site-packages/pynapple/core/metadata_class.py:217](http://localhost:8888/opt/homebrew/Caskroom/miniforge/base/envs/ccn-jan25/lib/python3.11/site-packages/pynapple/core/metadata_class.py#line=216), in _MetadataMixin.set_info(self, metadata, **kwargs)
    215         self._metadata[metadata.columns] = metadata
    216     else:
--> 217         raise ValueError("Metadata index does not match")
    218 elif isinstance(metadata, dict):
    219     # merge metadata with kwargs to use checks below
    220     kwargs = {**metadata, **kwargs}

ValueError: Metadata index does not match

[sjvenditto]

The problem is that integer indexing of metadata uses the pandas DataFrame .loc, which assumes the value is an existing index, which -1 is not. We can't switch to using .iloc directly because then it would break metadata indexing for TsGroup objects. We would have to treat integer/integer array indexing of metadata differently for IntervalSets and TsGroups

[gviejo]

Fix in #409