Fix VCN session units access

[danielsf]

The fix to the structure acronym handling broke access to the VCN release sessions.

from allensdk.brain_observatory.ecephys.ecephys_project_cache import (
    EcephysProjectCache)
import os
import hashlib
manifest_path = '/allen/aibs/informatics/danielsf/scratch/ecephys_manifest.json'
if os.path.exists(manifest_path):
    os.unlink(manifest_path)

cache = EcephysProjectCache.from_warehouse(manifest=manifest_path)
session = cache.get_session_data(session_id=719161530)
session.units

fails with

  File "test_warehouse.py", line 11, in <module>
    session.units
  File "/home/scott.daniel/AllenSDK/allensdk/core/lazy_property/lazy_property_mixin.py", line 13, in __getattribute__
    curr_attr = super(LazyPropertyMixin, self).__getattribute__(name)
  File "/home/scott.daniel/AllenSDK/allensdk/brain_observatory/ecephys/ecephys_session.py", line 227, in units
    return self._units.drop(columns=['width_rf',
  File "/home/scott.daniel/AllenSDK/allensdk/core/lazy_property/lazy_property_mixin.py", line 15, in __getattribute__
    return curr_attr.__get__(curr_attr)
  File "/home/scott.daniel/AllenSDK/allensdk/core/lazy_property/lazy_property.py", line 20, in __get__
    self.value = self.calculate()
  File "/home/scott.daniel/AllenSDK/allensdk/core/lazy_property/lazy_property.py", line 30, in calculate
    result = self.api_method(*self.args, **self.kwargs)
  File "/home/scott.daniel/AllenSDK/allensdk/brain_observatory/ecephys/ecephys_session_api/ecephys_nwb_session_api.py", line 158, in get_units
    isi_violations_maximum=self.isi_violations_maximum
  File "/home/scott.daniel/AllenSDK/allensdk/brain_observatory/ecephys/probes.py", line 114, in get_units_table
    ) for p in self.probes
  File "/home/scott.daniel/AllenSDK/allensdk/brain_observatory/ecephys/probes.py", line 114, in <listcomp>
    ) for p in self.probes
  File "/home/scott.daniel/AllenSDK/allensdk/brain_observatory/ecephys/_channels.py", line 60, in to_dataframe
    channels = [channel.to_dict()['channel'] for channel in self.value]
  File "/home/scott.daniel/AllenSDK/allensdk/brain_observatory/ecephys/_channels.py", line 60, in <listcomp>
    channels = [channel.to_dict()['channel'] for channel in self.value]
  File "/home/scott.daniel/AllenSDK/allensdk/core/_data_object_base/data_object.py", line 106, in to_dict
    properties = _get_keys_and_values(base_value=value)
  File "/home/scott.daniel/AllenSDK/allensdk/core/_data_object_base/data_object.py", line 89, in _get_keys_and_values
    for name, value in base_value._get_properties().items():
  File "/home/scott.daniel/AllenSDK/allensdk/core/_data_object_base/data_object.py", line 139, in _get_properties
    return {name: getattr(self, name) for name in props}
  File "/home/scott.daniel/AllenSDK/allensdk/core/_data_object_base/data_object.py", line 139, in <dictcomp>
    return {name: getattr(self, name) for name in props}
  File "/home/scott.daniel/AllenSDK/allensdk/brain_observatory/ecephys/_channel.py", line 78, in manual_structure_acronym
    if self._strip_structure_subregion \
AttributeError: 'float' object has no attribute 'split'

I do not think vbn_2022_dev before #2401 was merged.

This ticket is to diagnose and fix this problem so that we do not break access to the VCN 2019 data release.