Common function should be used in beta annealing in PER

[waffoo]

accel/accel/replay_buffers/prioritized_replay_buffer.py

Lines 42 to 44 in bf7f975

	# TODO replace it with common annealing function
	progress = min(1.0, self.steps / self.beta_steps)
	beta = self.beta0 + (1.0 - self.beta0) * progress