Coerce PMID column into string not integer

[sgsutcliffe]

In older versions of pandas (e.g. 2.1.0) the PMID column of Pointfinder Database was being converted to integer not the expected string. For example Klebsiella, was causing an error

File "lib/python3.11/site-packages/staramr/blast/pointfinder/PointfinderBlastDatabase.py", line 128, in get_cge_pmid
    return self._pointfinder_info.get_value(gene, mutation, "PMID")
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/lib/python3.11/site-packages/staramr/blast/pointfinder/PointfinderDatabaseInfo.py", line 206, in get_value
    results = ';'.join(matches[attribute])
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: sequence item 0: expected str instance, int found

[sgsutcliffe]

We will fix the error by coercing the PMID column into a string to avoid this error.