Add ridge/valley selection based on a morphological top-hat filter

[otto-link]

• ridges are extracted by the white top-hat
  • ridge = z - opening
• valleys are extracted by the black top-hat
  • valley = closing - z

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2001GL014412

• use disk or smooth kernels to avoid discontinuities