Non-string type attributes on nodes

[Wytamma]

I want to store non-string data on the nodes in a tree e.g. a matrix of numbers.

However, as the Node attributes are of type string | string[] I can't store the matrix (https://github.com/clockor2/phylojs/blob/main/src/tree/node.ts#L180). Would we be okay to change the type to include numbers or I can even see the utility of more complex datatypes.

You could then move the complexities of reading and writing these types of annotations to the custom user defined annotation readers/writers (https://github.com/clockor2/phylojs/blob/main/src/io/writers/newick.ts#L13).

Relates to #68 as we don't currently support user defined annotation parsers.

[LeoFeatherstone]

I added custom parsing in #86 and updated the docs accordingly.

Please note that square brackets are still hard coded as the open and close delimiters for annotations in Newick (eg. .../readers/newick.ts L188). Not sure how to get around this because we would need to pass it to the custom annotation function and somehow expose it before the function is called. I'm highly reluctant to pass any more arguments to readNewick otherwise.