Add optional "zoom" parameter to leastcostpath

[djohnson729]

This applies to both the command-line and python interfaces for least cost path. It should be an optional parameter that defaults to -1. A value of -1 means to run the LCP at the max zoom of the cost distance raster. An error should result if the specified zoom is beyond the max zoom level of the cost surface.

Also, update the map algebra reference online when complete.