`coveragecalculator`: Optimize File IO for coverage reports.

[ryanketzner]

The current process in the coveragecalculator class for writing coverage reports is inefficient.

            # compute coverage
            points = cov_checker.CheckPointCoverage() # list of indices of the GPs accessed shall be returned
            if len(points)>0: #If no ground-points are accessed at this time, skip writing the row altogether.
                for pnt in points:
                    coords = self.grid.get_lat_lon_from_index(pnt)
                    access_writer.writerow([time_index, pnt, coords.latitude, coords.longitude])

For every individual point accessed, the coordinates are requested one at a time. This requires a lot of python function calls and back and forth between python and C++. It would be better to request them all at once using something along the lines of self.grid.get_lats_lons_from_indices(pnts).