No default value for guppy_action_frequency_hz

The following code crashes because guppy_action_frequency_hz is None:

import robofish.gym_guppy
robofish.gym_guppy.envs.ConfigurableGuppyEnv(
    robot_action_frequency_hz=1.0,
    num_guppies=0,
)

• If num_guppies == 0, there should not be an error
• If num_guppies > 0, there should be a more informative error