Add function to produce NASC

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NASC (nautical area backscattering coefficients) is a high-level data product intimately related to biomass estimation. It can be viewed as a cleaned up form of the acoustic data (after removing noise, removing bottom, classifying regions of particular organisms, etc.) at regular spatial intervals along depth and geographical locations (say 10 m x 0.5 nmi "cells").

The hake survey uses PRC_NASC implementation in Echoview documented in this page for generating NASC: https://support.echoview.com/WebHelp/Reference/Algorithms/Analysis_variables/PRC_ABC_and_PRC_NASC.htm

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Use geopy to convert lat/lon to distance. @emiliom

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This is addressed in #1005. The remaining issues are documented in #1014.