Add a mosaic function

[tpoisot]

What the pull request does

Add a mosaic function to join multiple possibly overlapping layers

Type of change

Please indicate the relevant option(s)

• [x] :sparkle: New feature (non-breaking change which adds functionality)

Checklist

• [x] The changes are documented
  • [x] The docstrings of the different functions describe the arguments, purpose, and behavior
  • [x] There are examples in the documentation website
• [x] The changes are tested
• [x] The changes do not modify the behavior of the previously existing functions
• [x] The Project.toml field version has been updated

[mkborregaard]

He he, nice call.