fix/wallDistance

Noticed this bug just recently. In cases where there is a concave domain, the wall distance was overestimated.

|
|<...........>x
|           / ^
|_________ /  :
         |/   :
         |    :
         |    :
         |____v_______

previously, the wall distance for the location x would be the nearest distance to a wall along any axis ( .. or : line). The proper wall distance should be the distance to a wall, regardless which axis this wall is on (/ line).

Not sure how efficient this method is, but it works and does not decrease the speed of the simulations considerably.