Remove zone_letter and northern

[greatvovan]

Latitude bands are not part of UTM specification. Similarly, northing ranges for North and South hemispheres do not overlap. Therefore, zone_letter and northern parameters seem redundant and not needed for conversion.

It is okay however to output them when converting to UTM. But I think they should be removed from to_latlon() function.

https://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_system