Updates to events endpoint

[dlebauer]

• do not return eventParameters if they are null or empty string
• convert planting (plants / m2) --> planting w/ level and units
• convert fertilizer_X --> fertilization + type = 'X' (where X might be, e.g., N
• Only return observationunits for events that were not applied to the entire field (e.g. those that are related to a specific treatment)