title: Write .cti or .xml file in CANTERA
body: <p>I am trying to extract some skeletal mechanism from available detailed mechanism. 
  Here you can see extracting sub-mechanism from GRI3.0 mechanism. Is there any way to export the submechanism in CANTERA. More specifically can I somehow export gas2 in following example to a .cti or .xml file?</p>
  
  <pre><code>from cantera import *
  import numpy as np
  reaction_mech = 'gri30.cti'
  all_species = Species.listFromFile(reaction_mech)
  species = []
  # Filter species
  for S in all_species:
      comp = S.composition
  if 'C' in comp and comp.get('C') &gt;= 2:
      # Exclude all C compounds with more than 2 C atoms
      continue
  species.append(S)
  species_names = {S.name for S in species}
  print('Species: {0}'.format(', '.join(S.name for S in species)))
  all_reactions = Reaction.listFromFile(reaction_mech)
  reactions = []
  for R in all_reactions:
      if not all(reactant in species_names for reactant in R.reactants):
      continue
  if not all(product in species_names for product in R.products):
      continue
  reactions.append(R)
  gas1 = Solution('gri30.xml')
  gas2 = Solution(thermo='IdealGas', kinetics='GasKinetics',
                 species=species, reactions=reactions)
  </code></pre>