text: Title: Labelling while plotting time-series data over a network
  Body: Hi @marcomusy ,
  
  This is related to the question that was posted [here](https://github.com/marcomusy/vedo/issues/183).
  
  When I run the code below,
  ```
  
  import networkx as nx
  from vedo import *
  
  G = nx.gnm_random_graph(n=10, m=15, seed=1)
  nxpos = nx.spring_layout(G)
  
  nxpts = [nxpos[pt] for pt in sorted(nxpos)]
  nx_lines = [ (nxpts[i], nxpts[j]) for i, j in G.edges() ]
  
  nx_pts = Points(nxpts, r=12)
  nx_edg = Lines(nx_lines).lw(2)
  
  # node values
  values = [[1, .80, .10, .79, .70, .60, .75, .78, .65, .90],
            [3, .80, .10, .79, .70, .60, .75, .78, .65, .10],
            [1, .30, .10, .79, .70, .60, .75, .78, .65, .90]]
  time = [0.0, 0.1, 0.2] # in seconds
  for val,t in zip(values, time):
      nx_pts.cmap('YlGn', val, vmin=0.1, vmax=3)
      if t==0:
          nx_pts.addScalarBar()
   
      # make a plot title
      x0, x1 = nx_pts.xbounds()
      y0, y1 = nx_pts.ybounds()
      t = Text('My μ-Graph at time='+str(t)+' seconds',
               font='BPmonoItalics', justify='center', s=.07, c='lb')
      t.pos((x0+x1)/2, y1*1.4)
      show(nx_pts, nx_edg, nx_pts.labels('id',c='w'), t,
           interactive=True, bg='black')
  ```
  
  The `text` overlaps
  
  ![image](https://user-images.githubusercontent.com/29662579/111059761-0d7ed280-84be-11eb-88b2-4dd3b65b5f9f.png)
  
  I see this issue in the current version.
  
  Could you please suggest how to update the `text` that corresponds to each time instant and remove the `text` of the previous time step?
  
  