Master

1/ Patches the way assign_values_from_block is dealing with copy. It improves performances by a factor greater than two in several cases by reducing memory copies.

2/ Patches the resize_block method. When the current allocated size is greater than twice the size of the array, then shrink_to_fit is called to free memory. Previously it could have led to very huge memory allocation. In the case of a loop creating element one by one at the beginning of the mtv, memory consumption could be as high as number of element * size of original vector. This happens when using SubTotals function on very large spreadsheets.