Branch api

[granawkins]

This is the second of (now 3) big PRs; the first was component-api, the next will be engine-api.

In this one:

• Add Functions and Terminals as classes. They both have a .get() method which returns a list of the active nodes. This can include constants (for terminals), and will support logical operators by returning situation-specific functions with Operators.get(**kwargs).

• Add recursive Branch class. Tree is a wrapper for the the 'root' branch, stored in tree.root.

• Redo evolve methods to work with Branch. Evolution now uses a combination of Population methods (fitness_gene_pool, tournament) and Tree methods (copy, point_mutate, branch_mutate, crossover).

[granawkins]

Dev call 15-June:

1. Grant will fix the open comments
2. Resubmit PR, Ezio approve
3. Grant submit tiny PR with 3 changes and updated test results:
   • sigfig rounding
   • round classify outputs to 6 digits
   • replace tf.divide with tf.divide_no_nan
4. Grant submit engine-api with (updated) tests passing.