Klchan13 multi b

This rebases your multi_b branch on top of master. There are still a couple of test errors, both of them coming from the same bit of code: 

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    ERROR: osmosis.tests.test_mean_diffusivity_models.test_optimize_MD_params
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    Traceback (most recent call last):
      File "/home/arokem/usr/local/lib/python2.7/site-packages/nose-1.3.0-py2.7.egg/nose/case.py", line 197, in      runTest
        self.test(*self.arg)
      File     "/biac4/wandell/biac2/wandell6/users/arokem/projects/osmosis/osmosis/tests/test_mean_diffusivity_models.py", line 25, in test_optimize_MD_params
    mdm.decaying_exp, -0.5)
      File "/biac4/wandell/biac2/wandell6/users/arokem/projects/osmosis/osmosis/mean_diffusivity_models.py", line 192, in optimize_MD_params
        func = globals()[func_str]
    KeyError: -0.5

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    ERROR: osmosis.tests.test_mean_diffusivity_models.test_kfold_xval_MD_mod
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    Traceback (most recent call last):
      File "/home/arokem/usr/local/lib/python2.7/site-packages/nose-1.3.0-py2.7.egg/nose/case.py", line 197, in    runTest
        self.test(*self.arg)
      File     "/biac4/wandell/biac2/wandell6/users/arokem/projects/osmosis/osmosis/tests/test_mean_diffusivity_models.py   ", line 46, in test_kfold_xval_MD_mod
        -0.5, 10)
      File "/biac4/wandell/biac2/wandell6/users/arokem/projects/osmosis/osmosis/mean_diffusivity_models.py",     line 282, in kfold_xval_MD_mod
        func = globals()[func_str]
    KeyError: <function decaying_exp at 0x4802c80>

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