Parameters dropped when using subset parameter files

[m-philipps]

On branch develop: When supplying multiple parameter files to parameters.get_parameter_df, parameters will be dropped if they only differ from other parameters by their parameterId.

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Example using parameter files from demo_parameters.zip:

import petab
import pandas as pd

fp_params_1 = 'demo_parameters_1.tsv'
fp_params_2 = 'demo_parameters_2.tsv'

df = petab.parameters.get_parameter_df([fp_params_1, fp_params_2])
print(df.index.values)

Expected output:
['id11' 'id12' 'id13' 'id21' 'id22']

Actual output using develop at 5a87f83:
['id11' 'id21' 'id22']