Add percent_valid_points statistic to the get_stats() method in the Raster class

[vschaffn]

Context

Currently, the get_stats() method in the Raster class provides a set of statistics about the raster, such as minimum, maximum, mean, etc. However, there is no statistic that calculates the percentage of valid (non-NaN) points in the raster.
This feature would be particularly useful when working with DEMs.

Steps to implement

• Modify the get_stats() method to calculate the percentage of valid (non-NaN) points.
• Add this value to the dictionary returned by get_stats() under the key Valid points.

Note

This implementation will be carried out in geoutils.