Fixes for mono generator and use of relative paths

In this merge request the following fixes are implemented:

• fix to return empty dictionaries/arrays in load_events for the telescope which is not selected when using mono_event_generator. The problem was that it was trying to use empty arrays for the pointing interpolation (which is not needed anyway since no files are loaded for the not selected telescope)
• fix in find_mono_events when using mono_event_generator. The implementation was trying to use events from both telescopes, which is not possible when using this generator.
• fix to allow the usage of relative paths for input_url