feat: add DatabaseEmitter implementation class

[AlexPatrie]

What does this PR do?:

• feat: ports over and implements DatabaseEmitter class from vivarium-core, complete with all abstract methods in process-bigraph
• feat: adds utils functions to emitter.py file interacting with aforementioned object.
• docs: adds type annotations wherever possible, complete with doc string representation
• chore: refactors testing directory and associated files complete with test model files.
• feat: adds example processes to experiments library

[eagmon]

@AlexPatrie this is fantastic, thank you! Did you get this to work as-is?

One comment -- the Emitter base class is a type of Step and gets added to the Composite just like any other process would, so I think we need to add the schema method for this to work. Steps/Processes have inputs that they read and then perform the emit in whatever form. Here is an example of how an emitter gets wired into the composite:

process-bigraph/process_bigraph/experiments/minimal_gillespie.py

Lines 212 to 223 in 14d32a1

	'emitter': {
	'_type': 'step',
	'address': 'local:ram-emitter',
	'config': {
	'ports': {
	'inputs': {
	'mRNA': 'tree[float]',
	'interval': 'float'}}},
	'wires': {
	'inputs': {
	'mRNA': ['mRNA'],
	'interval': ['event', 'interval']}}}}}

[prismofeverything]

Good to get all this in here, thanks @AlexPatrie! Will need to be adapted a bit for the new emitter structure (emitters are just steps now) but it's a good start.

[eagmon]

This looks good, let's get the core processes out to a separate repo and then this is good to merge. Thanks!

[eagmon]

move to core-processes