Fix timepoint override check (#47)

* update using AMICI implementation

* fix replacement id construction

Author: FFroehlich

petab/core.py

@@ -139,8 +139,8 @@ def flatten_timepoint_specific_output_overrides(
         replacement_id = ''
         for field in possible_groupvars:
             if field in groupvars:
-                val = groupvar[groupvars.index(field)
-                               ].replace(';', '_').replace('.', '_')
+                val = str(groupvar[groupvars.index(field)
+                                   ]).replace(';', '_').replace('.', '_')
                 if replacement_id == '':
                     replacement_id = val
                 elif val != '':