Feature: Allow functions for node Fields

[IBlackI]

Pull request depends on #942

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/CirclonGroup/angular-tree-component/blob/master/CONTRIBUTING.md#commit-message-guidelines
• [x] Tests for the changes have been added (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Currently it is impossible to use the data of the node to programmatically determine the display. You can only use fixed values of the top level of the node data.

Resolves #793

What is the new behavior?

A function can be passed through the tree options which will evaluate the values for each node in the tree.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information