Zonal statistics: changed variance names (variables and field)

Zona Statistics is a great tool. However I propose a little change to make results of the computations clearer. There is a variable (an a table field in results) with name 'CV'. This field stores the variance rather than CV (CV stands for Coefficient of Variation). The Numpy method used to compute CV is "var". This Numpy method "returns the variance of the array elements". Thanks for your great work!