Simulation wizard: Add initial concentrations to initial conditions page

[hoanphungt]

@leendertvanwolfswinkel The initial conditions page contain 3 types of initial water levels: 1d, 2d and groundwater. Does this mean that there should be 3 upload boxes for substance concentrations respectively?

Screenshot:

[hoanphungt]

@leendertvanwolfswinkel I just realize that for the initial conditions page, we want to add initial concentrations, not substance concentrations to the page. So the implementation is different. I have some questions about initial concentrations. It seems like it needs to be added first before it can be used in the initial conditions page.

[leendertvanwolfswinkel]

@hoanphungt Daan has checked this and there is a small backend issue that will be fixed asap. Implementation in the plugin is fine, please merge