Allow user to select custom density of choosen material.

[grzanka]

When user selects given material and selects density dependendent units (like keV/um or SI lenght) allow the user to choose a custom value for density.

[Mexolius]

The problem is that rescaling from MeV*cm²/g to keV/um happens inside of libdedx function and it doesn't provide a way to specify the density. If we want to change the CSDA range using user-specified donsity we need to make sure the stps are consistent with this

[grzanka]

I agree that this requires adjusting libdedx as well