add tests for coverage.R for elevation, range subset

[maelle]

Reg range subset it depends on #79

[maelle]

we need test cases for elevation, dimensions.

[maelle]

and for nil values cf #86

[maelle]

Hoping the tutorials will have some use case I can steal. 😇 @annakrystalli (no pressure though)

[maelle]

For nil values, #121