title: Error when validating impact assessment method data and issues with regional characterization factors
body: <p>This question is somewhat related to <a href="https://stackoverflow.com/questions/44551595/create-a-new-method-in-brightway-2?newreg=a36ee6252cb34c57a7fa6a50d9d1ae49">this</a> one.</p>
  <p>I am attempting to validate the data for a new impact assessment method, prior to writing the method. The method data contains characterization factors for both global and regional interventions. I created a small toy example <a href="https://github.com/pbaustert/bw2_reg_cfs/" rel="nofollow noreferrer">here</a>.</p>
  <p>I am trying to validate the data as follows:</p>
  <pre><code>my_method = Method(('my method', 'a method', 'oh what a method'))
  
  method_data = [
      (('biosphere', 'global intervention'),1, u'GLO'),
      (('biosphere', 'regional intervention'),1, u'REG')
  ]
  
  my_method.validate(method_data)
  </code></pre>
  <p>The following error occurs:</p>
  <pre><code>MultipleInvalid: expected a list @ data[0]
  </code></pre>
  <p>No errors occur when attempting the write the method without validation. The error can be avoided by storing data in lists rather than tuples.</p>
  <p>Is this a bug in the package or am I doing something wrong?</p>
  <p>Furthermore, I am testing specifying regional identifiers for each characterization factors (as shown in the data above). This does not seem required, but when specifying an identifier other than <code>u'GLO'</code> the impacts are not accounted for in subsequent lca calculations. I test this in my <a href="https://github.com/pbaustert/bw2_reg_cfs/" rel="nofollow noreferrer">example notebook</a>.</p>
  <p>Should one avoid specifying regional identifiers for characterization factors?</p>