First implementation of tree package

Signed-off-by: p4u <p4u@dabax.net>

tree/README.md

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+#### Dvote Tree
+
+Implementation of dvote tree structure. Currently based on iden3 merkle tree.
+
+Example of usage:
+
+```
+  T := tree.Tree {namespace: "vocdoni"}
+  if T.init() != nil { fmt.Println("Cannot create tree database") }
+  err := T.addClaim([]byte("Hello you!"))
+  if err != nil {
+    fmt.Println("Claim already exist")
+  }
+  mpHex, err := T.genProof([]byte("Hello you!"))
+  fmt.Println(mpHex)
+  fmt.Println(T.checkProof([]byte("Hello you!"), mpHex))
+  T.close()
+```
+
+### To-Do
+
++ Add export/import methods

tree/tree.go

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+package tree
+
+import (
+  "github.com/iden3/go-iden3/db"
+  "github.com/iden3/go-iden3/merkletree"
+  mkcore "github.com/iden3/go-iden3/core"
+  common3 "github.com/iden3/go-iden3/common"
+  "os/user"
+)
+
+type Tree struct {
+    namespace string
+    storage string
+    tree *merkletree.MerkleTree
+}
+
+func (t *Tree) init() error {
+  if len(t.storage) < 1 {
+    usr, err := user.Current()
+    if err == nil {
+      t.storage = usr.HomeDir + "/.dvote/Tree"
+    } else { t.storage = "./dvoteTree" }
+  }
+  mtdb, err := db.NewLevelDbStorage(t.storage, false)
+	if err != nil {
+    return err
+	}
+	mt, err := merkletree.New(mtdb, 140)
+	if err != nil {
+	  return err
+	}
+  t.tree = mt
+  return nil
+}
+
+func (t *Tree) close() {
+	defer t.tree.Storage().Close()
+}
+
+func (t *Tree) addClaim(data []byte) error {
+  claim := mkcore.NewGenericClaim(t.namespace, "default", data, nil)
+  return t.tree.Add(claim)
+}
+
+func (t *Tree) genProof(data []byte) (string, error) {
+  claim := mkcore.NewGenericClaim(t.namespace, "default", data, nil)
+  mp, err := t.tree.GenerateProof(claim.Hi())
+  if err!=nil {
+    return "", err
+  }
+  mpHex := common3.BytesToHex(mp)
+  return mpHex, nil
+}
+
+func (t *Tree) checkProof(data []byte, mpHex string) (bool, error) {
+  mp, err := common3.HexToBytes(mpHex)
+  if err != nil {
+    return false, err
+  }
+  claim := mkcore.NewGenericClaim(t.namespace, "default", data, nil)
+  return merkletree.CheckProof(t.tree.Root(), mp, claim.Hi(), claim.Ht(), t.tree.NumLevels()), nil
+}