Update equation that calculates probability of dispersal in gridded mode

[kpalmqui]

Current implementation:

float _probabilityOfDispersal(float rate, float height, float distance) {
       return exp(rate / sqrt(height)) * distance;
}

Proposed implementation:

float _probabilityOfDispersal(float rate, float height, float distance) {
  return exp(rate * distance/ height);
}

[kpalmqui]

resolved by ee338a4