Fix events reader and references instrument ids

Events related changes

• Add csv OPL syntax to the default read_events() method (closes #112 (closed))
• Add new keys (start_time, end_time) for EventWindow() definition (#112 (closed))
• Fix ITL reader with explicit milliseconds in datetime (closes #109 (closed))
• Fix CSV reader with an uncommented header (closes #108 (closed))

Spice reference changes

• Fix SpiceInstrument with invalid NAIF id references (closes #119 (closed))

Miscellaneous

• Add explicit slug to RTD API call during CI.