BonePosFinder added leg third joint interpolation.

When searching for the first three joint positions if it find two then it tries
to interpolate the third.
CommonFitter added method to solve chains ignoring the end result.

Some times is useful simply to try and put the chain in a given position
even though it will not reach it.
LegFitter added approximate method for leg fitting.

Of the paw is placed, we have calculated a previous joint position and it fails
to put it there. Then try and put previous bone in a close position without moving
paw bone.