Introduce CV_COMP_CHISQR_ALT, an alternative method to calculate ChiSquare Histogram comparison

There's some disagreement about the correct formula.
has its supporters, however, for texture analysis, the newly introduced formula became
standard. The commit enables both uses without breaking backward compatibility.

First contributor of this commit was sperrholz.
Merge pull request #1248 from vkocheganov:chi_square_alt