add stats sfs

__init__.py

-from frst import sfs_tools, customgraphics, vcf_utils, sfs_tools
+from frst import sfs_tools, customgraphics, vcf_utils, sfs_tools, stats_sfs

customgraphics.py

     if max_chr_id :
         nb_iter = max_chr_id
     else:
-        nb_iter =  len(recent_variants)
+        nb_iter =  len(recent_variants) -1
     if show :
         iter_start = min_chr_id + 1
         if not step :

stats_sfs.py

+from frst import vcf_to_sfs
+
+import math
+
+if __name__ == "__main__":
+
+
+    sfs_folded_hir = {0: 575935, 1: 346313, 2: 220539, 3: 130150, 4: 69989, 5: 33768, 6: 41031}
+
+    n = len(sfs_folded_hir.values())
+    pi = 0
+    s_prime = 0
+    for i in range(1, n-1):
+        pi += i * (n-1) * sfs_folded_hir[i-1] / math.comb(n, 2)
+        #s_prime +=  math.comb() * math.comb() / math.comb()
+
+
+    print(pi)
+
+
+