Changing colormaps in GMT documentation figures

Many figures in the GMT documentation do not use perceptually uniform colormaps (e.g., rainbow in [the tutorial](https://docs.generic-mapping-tools.org/dev/tutorial/session-4.html#color-images)). 260+ images in the GMT docs and tests will be updated as part of [this PR](https://github.com/GenericMappingTools/gmt/pull/3344). Should we take this opportunity to update the scripts to use 'scientifically derived color maps' based on the research presented in [Crameri et al. (2020)](https://www.nature.com/articles/s41467-020-19160-7.epdf?sharing_token=R_K5bGfsequg9SzBpaczA9RgN0jAjWel9jnR3ZoTv0N87NSBYatlOY6U8p_acnc1ZqadIZ2dXsaV7zhtZeQ7KToucB-_XaWXBNc7BY-1Tj8JCm3s1--oDCgQ3FJ3GYh8OO3wRjmc0_Skz1Ks6wO5LbgCh92B-z5cPGA7DM1A2Ac%3D)?