Index the x column when converting Corsika to hdf5
Add warnings to Corsika reader

Improve some docstrings
Correct spacing around docstrings
Add cleanup function

Easily close the Corsika file when you are done with the simulation
Add simulate gps function to get an ext_timestamp

Use the shower_id as the timestamp in seconds.
Use the arrival time of the second high signal as the nanosecond part
of the ext_timestamp