preEventSummary: Pre-event summary
In hydroEFS: Hydrological Event Finding and Summarising (hydroEFS).

This function provides a summary of data of a given period before each event.

1	preEventSummary(eventCount, data, stepsBefore)

`eventCount`	A vector indicating events. The `eventCounts` of `findEvents`.
`data`	A vector of data (e.g rainfall, stream flow, temperature) of length(eventCounts)
`stepsBefore`	The amount of temporal steps as the begining of the window to summarise (`start_of_event-stepsBefore:start_of_event`).

`minimum`	The minumum of the event
`mean`	The mean of the event
`median`	The median of the event
`std_dev`	The standard deviation of the event
`variance`	The variance of the event