Percentage Exceedence of a 3D Matrix for a Threshold value

This function calculates the percentage of time that a given 3D Matrix exceeds some threshold value.

It counts each point along the 1st Dimension that exceeds the threshold and expresses this as a percentage of the length of the 1st Dimension. This is repeated across the 2nd and 3rd Dimensions. The result is a 2D matrix with size identical to the 2nd and 3rd Dimensions that contains percentage figures.

Brief example of where it was used:
We had gridded model data which contained concentrations at a specific depth, of metals in Table Bay Harbour, for a specific time period. We needed to know for what percentage of the time those concentrations exceeded the value prescribed by the Water Quality Guidelines. The exceedence function was written specifically for this purpose, but has been modified to be used in other situations