Remove 0-index compensation in scale calculation
Remove 0-index compensation in scale calculation

Lengths aren't 0-indexed so we don't have to subtract one from
axisLength when computing averageScale(0, axisLength): for a lengt of 1
averageScale(0, 1) produces average scale over one unit of length, as
desired.

Thanks @BobHanson!

Closes #408

scifio/scifio