remove deprecated string class method call

[G-Thor]

Replaced a call to a deprecated string method which caused the code to fail in python 3.
Only line 259 of src/logplot/logging_plotting.py is modified

Here is an example of a stack trace

File "./merlin/src/run_merlin.py", line 1320, in
main_function(cfg)
File "./merlin/src/run_merlin.py", line 870, in main_function
cmp_mean_vector = cmp_mean_vector, cmp_std_vector = cmp_std_vector,init_dnn_model_file=cfg.start_from_trained_model)
File "./merlin/src/run_merlin.py", line 382, in train_DNN
plotlogger.save_plot('training convergence',title='Progress of training and validation error',xlabel='epochs',ylabel='error')
File "/xxx/merlin/src/logplot/logging_plotting.py", line 259, in save_plot
filename = self.plot_path + "/" + string.replace(plot_name, " ", "_") + ".pdf"
AttributeError: module 'string' has no attribute 'replace'