Investigate: test issue #339 with file paths containing "_".

[priscavdsluis]

Software support contacted us with the following report (translated):

• We checked the mdu file for case sensitivity. The file names are available exactly as they are in the model.
• We went through the file names in the Sobek_3b.fnm and made all file names in the model the same.
• We have also tried the HYDROLIB core functionality for the above 2 points (https://deltares.github.io/HYDROLIB-core/0.5.2/tutorials/loading_and_saving_a_model/#loading-models-on-case-sensitive-systems), but that checks more and then you are not allowed to have an id like 'pav_2' because '_' is not a decimal sign. That was a bit too strict for us.