Clarify selection of events in EM-DAT

[stufraser1]

The event frequency charts show different data for total damage and people affected.
This is because the events in EM-DAT data catalog are selected based on whether there is a value present for total damage and people affected.
Typically, more events for a country and peril will have a value for people affected, so more events will appear in the chart.

We need to make this clearer in the guidance and the tool that the frequency charts are showing number of events for which there is a total damage or pop affected value (not the total number of events in the period and for the peril.

[stufraser1]

• UI: added text "Events are shown only if the impact metric is non-zero in the historical loss catalogue. It is common for eevnts to have a zero loss for Economic Damage and non-zero loss for people affected - therefore the number of events displayed will differ by impact metric selected."

• To be reflected in user guide.