bugfix: correction of the nb_sim specification in the heatpde-sa server parameter generator

The parameter_generator was using nb_sim=self.study_options['num_samples'] instead of self.study_options['parameter_sweep_size'].

This MR fixes it.

Note: this was working because self.study_options['num_samples']>self.study_options['parameter_sweep_size'] in all tests.

Note: this error is not present in the heatpde_dl_server.py. As evident from this MR's bug, the variable name num_samples seems highly confusing since there already an entry with this name in the config file.