Added a new heuristic that uses gradient maps and a new solver that
stopps the time another solver needs for the execution in total.
Merge branch 'master' of https://github.com/milseman/sokoban.git
Merge branch 'master' of https://github.com/milseman/sokoban.git
Added a new heuristic with gradient maps from the goals based on the
static board. Also added an execution environment that compares
algorithms.