turn callback helpers in ODReader to decorators

Current:

from pioreactor.background_jobs.od_reading import ODReader

# example of using callbacks to OD reading...

def hello(self):
    print(f"Hello, {self}!")


def goodbye(self, readings):
    print(f"Goodbye, {self}, witnessed {readings}!")

ODReader.add_pre_read_callback(hello)
ODReader.add_post_read_callback(goodbye)

Desired:

from pioreactor.background_jobs.od_reading import ODReader

# example of using callbacks to OD reading...

@ODReader.add_pre_read_callback
def hello(self):
    print(f"Hello, {self}!")


@ODReader.add_post_read_callback
def goodbye(self, readings):
    print(f"Goodbye, {self}, witnessed {readings}!")