Add a Computation abstract class

[khaeru]

This can be the location for:

• add_task(c: Computer) or similar for describing computations in c.
• __call__(): the actual callable to be executed.
• __repr__(): a more readable string representation for Computer.describe().
• etc.

These should be easier to maintain if they are collected, instead of the separate pair of e.g. Computer.convert_pyam (for adding task(s)) and .compat.pyam.computations.as_pyam (the actual callable)

This will also alow to reduce complexity of this code in Computer.add():

genno/genno/core/computer.py

Lines 144 to 162 in 43a1702

	elif isinstance(data, str) and self.get_comp(data):
	# *data* is the name of a pre-defined computation
	name = data
	if hasattr(self, f"add_{name}"):
	# Use a method on the current class to add. This invokes any
	# argument-handling conveniences, e.g. Computer.add_product()
	# instead of using the bare product() computation directly.
	return getattr(self, f"add_{name}")(*args, **kwargs)
	else:
	# Get the function directly
	func = self.get_comp(name)
	# Rearrange arguments: key, computation function, args, …
	func, kwargs = partial_split(func, kwargs)
	return self.add(args[0], func, *args[1:], **kwargs)
	elif isinstance(data, str) and data in dir(self):
	# Name of another method, e.g. 'apply'
	return getattr(self, data)(*args, **kwargs)

The Computer can:

• Look up the Computation class in Computer.modules.
• If it has an add_task() method, call that directly; else, simply instantiate.