Hard coded upper frequency limit could be removed

[f-brinkmann]

In Mesh2HRTF v1.2.1 https://github.com/Any2HRTF/Mesh2HRTF/blob/master/mesh2hrtf/Mesh2Input/mesh2input.py#L154 the upper frequency limit for numerical simulations is set to 96 kHz. The idea behind this was to somhow limit the number of required elements in the mesh due to restrictions in NumCalc. However, this might not always be required, depending on the size of the mesh. It might be better to issue a warning, if the Mesh has too many elements for NumCalc to handle.