apply HVSR to virtual borehole code for f0 values other than stored in the .hv file

Geopsy stores HV results in the .hv files. The code currently only calculates the bedrock depth for f0 values stored in the .hv values. If a wrong f0 value is stored in the .hv file, the code returns a wrong graph (e.g. a 40Hz peak).

Solution would be an option to search in a frequency range of interest for the researcher.