Add union to merge methods

[moradology]

Overview

This PR adds a tile merging function, union which combines two input tiles so as to preserve all non-overlapping regions.

Checklist

• [x] ./CHANGELOG.md updated, if necessary. Link to the issue if closed, otherwise the PR.
• [x] New user API has useful Scaladoc strings
• [x] Unit tests added for bug-fix or new feature

Demo

Union of two tiles; tile 1 with extent 0,0,1,1 and tile2 with extent 2,2,2.2,2.2. Output tile with extent of 0,0,2.2,2.2:

Union of overlapping tiles with a function applied to average overlapping cell values:

Closes #3365