Improve probability calculation in Square::probability()

[meltedlilacs]

At the moment the probability of there being something in a Square is simply numTimesFound / numTimesScanned, not taking into account the average number of other Square's scanned at the same time.

The formula should be (numTimesFound / numTimesScanned) * (1 / error rate), where error rate is the average number of Square's scanned at once when this Square was scanned.

What is the best method to calculate the error rate? I think the method we came up with has to do with checking adjacent Square's to see if they have been scanned from the same place, but we must be careful to check only the ones the same distance from the origin of the scan, and only up to 15 degrees away.