Add GDALViewshedGenerate binding

[pericles-tpt]

• implement GDALViewshedGenerate binding
• implement Viewshed go function (calls binding)
• add TestViewshedSimpleHeight to test the binding

[pericles-tpt]

Question: Invalid test results across GDAL versions

I found in my testing that different versions of GDAL pass different sections of the test:

1. 3.1.0 < version < 3.4.2 - doesn't pass any tests
2. 3.4.2 <= version < 3.10.1 - passes all tests EXCEPT when output mode is DEM
3. 3.10.1+ - passes all tests

My solution for this is to return an error for any case where tests produce an invalid result, the current behaviour for the above is:

1. Returns an error at the start of Viewshed()
2. Returns an error IF the output mode is DEM
3. Runs Viewshed() as normal

Do you prefer the approach I've implemented, which prevents the user from running Viewshed() when it could produce invalid data? Or, should I add a warning in the Viewshed() documentation comment and remove the errors for cases (1) and (2), this would allow the user to run Viewshed() for GDAL version 3.1.0+ at their own risk?

[coveralls]

coverage: 98.403% (-0.1%) from 98.548%
when pulling f67b649 on Arlula:viewshed
into 3ca5b20 on airbusgeo:main.