Add treeinterface benchmarks

```
nCPU: 4, nLeafs: 10000
	asmtree.Add loop:	1m33.393613916s
	asmtree.AddBatch:	480.164271ms
	gravitontree.Add loop:	10.127451689s
	gravitontree.AddBatch:	141.29108ms
	arbo.Add loop:		1.16466211s
	arbo.AddBatch:		97.870759ms

nCPU: 8, nLeafs: 10000
	asmtree.Add loop:	24.538163095s
	asmtree.AddBatch:	194.14454ms
	gravitontree.Add loop:	6.150883978s
	gravitontree.AddBatch:	90.742666ms
	arbo.Add loop:		600.259558ms
	arbo.AddBatch:		50.440291ms

---

nCPU: 4, nLeafs: 500000
	asmtree.AddBatch:	24.95076993s
	gravitontree.AddBatch:	11.768588412s
	arbo.AddBatch:		23.360363426s

nCPU: 8, nLeafs: 500000
	asmtree.Add loop:	(not executed)
	asmtree.AddBatch:	14.363474201s
	gravitontree.Add loop:	8m45.300930089s
	gravitontree.AddBatch:	7.377581508s
	arbo.Add loop:  	2m0.229851504s
	arbo.AddBatch:  	8.759028338s
```

treeinterface/benchmarks/bench_test.go

@@ -0,0 +1,94 @@
+package benchmarks
+
+import (
+	"crypto/rand"
+	"fmt"
+	"runtime"
+	"testing"
+	"time"
+
+	"github.com/arnaucube/kesto/treeinterface/arbotree"
+	asmtree "github.com/p4u/asmt"
+	"go.vocdoni.io/dvote/censustree"
+	"go.vocdoni.io/dvote/censustree/gravitontree"
+)
+
+// func BenchmarkAddBatch(b *testing.B) {
+func TestAddBatch(t *testing.T) {
+	nLeafs := 100_000
+	fmt.Printf("nCPU: %d, nLeafs: %d\n", runtime.NumCPU(), nLeafs)
+
+	// prepare inputs
+	var ks, vs [][]byte
+	for i := 0; i < nLeafs; i++ {
+		k := randomBytes(32)
+		v := randomBytes(32)
+		ks = append(ks, k)
+		vs = append(vs, v)
+	}
+
+	tree1 := &asmtree.Tree{}
+	benchmarkAdd(t, "asmtree", tree1, ks, vs)
+
+	tree1 = &asmtree.Tree{}
+	benchmarkAddBatch(t, "asmtree", tree1, ks, vs)
+
+	tree2 := &gravitontree.Tree{}
+	benchmarkAdd(t, "gravitontree", tree2, ks, vs)
+
+	tree2 = &gravitontree.Tree{}
+	benchmarkAddBatch(t, "gravitontree", tree2, ks, vs)
+
+	tree3 := &arbotree.Tree{}
+	benchmarkAdd(t, "arbo", tree3, ks, vs)
+
+	tree3 = &arbotree.Tree{}
+	benchmarkAddBatch(t, "arbo", tree3, ks, vs)
+}
+
+func benchmarkAdd(t *testing.T, name string, tree censustree.Tree, ks, vs [][]byte) {
+	storage := t.TempDir()
+	err := tree.Init("test1", storage)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	start := time.Now()
+	for i := 0; i < len(ks); i++ {
+		if err := tree.Add(ks[i], vs[i]); err != nil {
+			t.Fatal(err)
+		}
+	}
+	printRes(t, name+".Add loop", time.Since(start))
+}
+
+func benchmarkAddBatch(t *testing.T, name string, tree censustree.Tree, ks, vs [][]byte) {
+	storage := t.TempDir()
+	err := tree.Init("test1", storage)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	start := time.Now()
+	invalids, err := tree.AddBatch(ks, vs)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(invalids) != 0 {
+		t.Fatal("len(invalids)!=0")
+	}
+	printRes(t, name+".AddBatch", time.Since(start))
+}
+
+func printRes(t *testing.T, name string, duration time.Duration) {
+	fmt.Printf("	%s:	%s \n", name, duration)
+}
+
+func randomBytes(n int) []byte {
+	b := make([]byte, n)
+	_, err := rand.Read(b)
+	if err != nil {
+		panic(err)
+	}
+	return b
+}

treeinterface/benchmarks/go.mod

@@ -0,0 +1,12 @@
+module github.com/arnaucube/kesto/treeinterface/benchmarks
+
+go 1.16
+
+replace github.com/arnaucube/kesto/treeinterface/arbotree => ../arbotree
+
+require (
+	github.com/arnaucube/arbo v0.0.0-20210519105811-13378d338ef2
+	github.com/arnaucube/kesto/treeinterface/arbotree v0.0.0-20210518090017-aff9bcc18604
+	github.com/p4u/asmt v0.0.0-20210327173007-3e47da54b6b6
+	go.vocdoni.io/dvote v0.6.1-0.20210519145433-4b6da3324284
+)