Colored tree improvements

[ranocha]

The first version is implemented in #54. Left to do:

• General ColoredRootedTreeIterator
• LaTeX output of (bi-) colored rooted trees
  • This could be based on https://tex.stackexchange.com/a/673436
  • new latexify setup to support colored trees #100
• filter function to filter out some trees for the iterators, e.g., for Nyström methods
• Possible performance improvements are marked with # TODO: ColoredRootedTree (colored trees performance #71)
• partitions (partitions of colored trees #69) (required for substitute)
• SplittingIterator (required for compose)

See also #28 and ranocha/BSeries.jl#8