Draft: Use one bin in CavityResonator

I wanted to run a simulation using only one particle and one bin per bunch but discovered that the minimum number of bins I could use with the CavityResonator is 3 because of how the binning and length of a bin is calculated.

These are the changes I had to do to be able to run with n_bin=1 but maybe changing the binning like this could have consequences at many other places?