title: why biosphere activities may appear in the technosphere matrix?
body: <p>I was testing how to add a new &quot;activity&quot; to my biosphere3 matrix in brightway, and I got a new column in my technosphere matrix that I was not expecting, (corresponding to the id of my new biosphere node). Is this behaviour the expected one ?. Not that I want to stick to traditions, but usually columns in the technosphere matrix are not &quot;elementary flows&quot;. I am using bw2calc version (2.0.DEV1) and bw2data version (4.0.DEV1)</p>
  <p>Here some code to try to reproduce this behaviour:</p>
  <pre><code>b3 = bw2data.Database('biosphere3')
  
  uwva_node = b3.new_activity(code='uwva_ppp',
              name='utility weighted value added (PPP)',
              categories=('social',),
              unit = 'Meuro_ppp',
              )
  uwva_node.save()
  </code></pre>
  <pre><code>act1_key=('test_db','activity_1')
  
  biosphere_exchange_1={'amount':2,
                      'input':uwva_node.key,
                      'output':act1_key,
                      'type':'biosphere',
                      'uncertainty type': 0}
  
  production_exchange_1={'amount':2,
                       'input':act1_key,
                       'output':act1_key,
                       'type':'production',
                       'uncertainty type':0}
  
  act_1_dict={'name':'test_activity_1',
   'unit':'megajoule',
   'exchanges':[production_exchange_1,biosphere_exchange_1]}
  
  act2_key=('test_db','activity_2')
  
  production_exchange_2={'amount':10,
                       'input':act2_key,
                       'output':act2_key,
                       'type':'production',
                       'uncertainty type':0}
  
  technosphere_exchange_1={
      'amount':10, # 
      'input':act1_key,
      'output':act2_key,
      'type':'technosphere',
  }
  
  act_2_dict={'name':'test_activity_2',
              'unit':'megajoule',
              'exchanges':[production_exchange_2,technosphere_exchange_1]}
  
  database_dict={act1_key:act_1_dict,
                 act2_key:act_2_dict}
  
  db=bw2data.Database('test_db')
  
  db.write(database_dict)
  </code></pre>
  <p>if I calculate the inventory I get a technosphere matrix with 3 columns. And my new biosphere activity is on the technosphere matrix as well.</p>
  <pre><code>act2 = bw2data.get_activity(act2_key)
  
  lca = bw2calc.LCA({act2:1})
  
  lca.lci()
  
  lca.technosphere_matrix.todense()
  
  assert uwva_node.id in lca.dicts.activity
  assert uwva_node.id in lca.dicts.biosphere
  </code></pre>