Quickfix min/max elevation for new SRTM15+V2 earth relief grids

[weiji14]

Upstream GMT has updated its earth relief grids (see GenericMappingTools/gmt#1846) to use the new Global Bathymetry and Topography at 15 Arc Sec (SRTM15+V2) data by @btozer and others!

References:

• Tozer, B., Sandwell, D. T., Smith, W. H. F., Olson, C., Beale, J. R., & Wessel, P. (2019). Global Bathymetry and Topography at 15 Arc Sec: SRTM15+. Earth and Space Science. https://doi.org/10.1029/2019EA000658

Description of proposed changes

This pull request fixes the test failures by changing the minimum/maximum elevation values to that of the new earth relief grids.

Fixes #

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to docstrings or tutorials.

[weiji14]

Hmm, I would have thought there would be more tests (e.g. from grdimage) failing. Is the @pytest.mark.mpl_image_compare decorator not doing its job properly?!!

[seisman]

The grdimage tests use the lowest resolution (60m) earth relief data. I think we can't see any difference between the old and new dataset for such low resolution.

[weiji14]

Yes, but surely there would be some?? I'll merge this in first and investigate later.