Allow c8_air_shower to sample from energy range

Issues: Closes #643 (closed)

This implements the ability to sample the primary energy from a power law. The original interface is still allowed, --energy is no longer required. Instead, --emin and --emax can be given along with a power law index --eslope.

For the most part, this is just a rearrangement of the script to put the right things inside the for-loop.

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