Compute Mean Volume Backscattering Strength (MVBS)
based on intervals of range_sample and ping number (ping_num) specified in index number.
Output of this function differs from that of compute_MVBS, which computes
bin-averaged Sv according to intervals of range (echo_range) and ping_time specified
in physical units.
dataset containing Sv and echo_range [m]
number of samples to average along the range_sample dimension, default to 100
number of pings to average, default to 100