to_dataframe fails if the number of groups is not == 1 #137
Description
when len(self.groups) != 1, the following error occurs:
File "/home/dana.mastrovito/.local/lib/python3.9/site-packages/sonata/circuit/population.py", line 245, in to_dataframe
dataframes.append(self.get_group(grp_id)).to_datframe()
AttributeError: 'NoneType' object has no attribute 'to_datframe'
On line 245, it appears that the parentheses cause to_datframe() to be called on the result of dataframes.append(...), which is None, rather than on the group object. This seems to attempt to convert a list to a dataframe.
On the next line (246), the code tries to set an index on what appears to still be a list—possibly indicating that the intended behavior was to concatenate the dataframes first.
There also seems to be a typo: .to_datframe should be .to_dataframe