extract_profiles.py: multiprocessing version bails

[aaschwanden]

Running extract_profiles.py -a -n 2 ..... bails with

Traceback (most recent call last):
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/bin/extract_profiles.py", line 1415, in <module>
    p.map(extract, vars_list)
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/multiprocessing/pool.py", line 251, in map
    return self.map_async(func, iterable, chunksize).get()
  File "/opt/local/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/multiprocessing/pool.py", line 567, in get
    raise self._value
RuntimeError: NetCDF: HDF error
  - done with climatic_mass_balance_cumulative
extract_variable function took 0.030 s
  Reading variable dbdt
    - processing profile C1

This has low priority.