Add thresholdNLeafs configurable

ThresholdNLeafs defines the threshold number of leafs in the tree that
determines if AddBatch will work in memory or in disk. It is defined
when calling NewTree, and if set to 0 it will work always in disk.

hash.go

@@ -77,7 +77,9 @@ func (f HashPoseidon) Len() int {
 	return 32 //nolint:gomnd
 }
 
-// Hash implements the hash method for the HashFunction HashPoseidon
+// Hash implements the hash method for the HashFunction HashPoseidon. It
+// expects the byte arrays to be little-endian representations of big.Int
+// values.
 func (f HashPoseidon) Hash(b ...[]byte) ([]byte, error) {
 	var toHash []*big.Int
 	for i := 0; i < len(b); i++ {