get_tkblattnr - Calculates the sheetnumber of TK25 maps from latitude and longitude.
This function calculates from given geografic coordinates [decimal degree] the relevant topographic map (Topographische Karte TK25)
The size of a TK25 map is defined as 10 geografic minutes in width and 6 geografic minutes in height.
(matrix aware)
no warranty, there are no input checks.
get_tkblattnr(11.56, 48.14)