neurom.load_morphologies does not resolve paths

[eleftherioszisis]

Works:

import neurom
pop = neurom.load_morphologies("tests/data/valid_set")
[m.name for m in pop]

Fails:

import neurom
pop = neurom.load_morphologies("tests/data/valid_set")
cd ..
[m.name for m in pop]

with:

RawDataError                              Traceback (most recent call last)
File ~/Projects/NEUROM/NeuroM/neurom/core/population.py:87, in Population._load_file(self, f)
     86 try:
---> 87     return neurom.load_morphology(f)
     88 except (NeuroMError, MorphioError) as e:

File ~/Projects/NEUROM/NeuroM/neurom/io/utils.py:168, in load_morphology(morph, reader)
    166     return Morphology(_get_file(morph, reader))
--> 168 return Morphology(morph, Path(morph).name)

File ~/Projects/NEUROM/NeuroM/neurom/core/morphology.py:516, in Morphology.__init__(self, filename, name)
    510 """Morphology constructor.
    511 
    512 Args:
    513     filename (str|Path): a filename or morphio.{mut}.Morphology object
    514     name (str): an optional morphology name
    515 """
--> 516 self._morphio_morph = morphio.mut.Morphology(filename).as_immutable()
    517 self.name = name if name else 'Morphology'

RawDataError: File: tests/data/valid_set/Neuron_slice.h5 does not exist.

The above exception was the direct cause of the following exception:

NeuroMError                               Traceback (most recent call last)
Input In [8], in <cell line: 1>()
----> 1 [m.name for m in pop]

Input In [8], in <listcomp>(.0)
----> 1 [m.name for m in pop]

File ~/Projects/NEUROM/NeuroM/neurom/core/population.py:98, in Population.__iter__(self)
     96 """Iterator to populations's morphs."""
     97 for f in self._files:
---> 98     m = self._load_file(f)
     99     if m is None:
    100         continue

File ~/Projects/NEUROM/NeuroM/neurom/core/population.py:92, in Population._load_file(self, f)
     90         L.info('Ignoring exception "%s" for file %s', e, f.name)
     91     else:
---> 92         raise NeuroMError('`load_morphologies` failed') from e
     93 return None

NeuroMError: `load_morphologies` failed

Reported by @lidakanari