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.

testvectors/circom/go-data-generator/generator_test.go

@@ -16,7 +16,8 @@ func TestGenerator(t *testing.T) {
 	c := qt.New(t)
 	database, err := badgerdb.New(db.Options{Path: c.TempDir()})
 	c.Assert(err, qt.IsNil)
-	tree, err := arbo.NewTree(database, 4, arbo.HashFunctionPoseidon)
+	tree, err := arbo.NewTree(arbo.Config{Database: database, MaxLevels: 4,
+		HashFunction: arbo.HashFunctionPoseidon})
 	c.Assert(err, qt.IsNil)
 
 	testVector := [][]int64{