Define registration system for Python algorithms for prototype 0.1

[knoepfel]

The expected registration capabilities of Python algorithms depends largely on further discussion with DUNE. What is required for prototype 0.1 is a demonstration that Python algorithms can be registered with Phlex using one possible registration scheme.

[wlav]

The current implementation follows the C++ one closely. Example:

from phlex.registry import register

def add(i: int, j: int) -> int:
    return i + j

def PHLEX_EXPERIMENTAL_REGISTER_ALGORITHMS(m, config):
    register(add, m, config)

This will remain the philosophy going forward to guarantee that similar coding patterns can be written (e.g. registering multiple algorithms and allowing registration to be tweaked based on configuration) and to ensure that any C++ developer will be able to understand, at a glance, what the Python registration does and vv.