This function adds up all block lengths (number of SNPs in each SNP blocks), which are stored in the names along the third dimension of the array. When the f2-statistics were computed while setting `maxmiss`

to values greater than 0, it is possible that not all f2-statistics are based on the same number of SNPs. In that case, the value returned by this function is merely an upper bound on the number of SNPs used.

`count_snps(f2_blocks)`

- f2_blocks
A 3d array of per block f2-statistics

The total number of SNPs across all blocks