Ability to lock DotDicts to prevent accidental modifications of core 'constants'

This patch adds a function to DotDict that allows you to lock its contents (stays in effect until you `clear()` it).  

This patch applies the functionality to PRED, RT, LCE, and so on.  If you try to overwrite their attributes, you get an error:

```
><> from pyon.public import RT

><> RT.InstrumentDevice
--> 'InstrumentDevice'

><> RT.InstrumentDevice = 3
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
/Users/iankatz/.virtualenvs/artoo/sdtkiai/coi-services/extern/pyon/scripts/pycc.pyc in <module>()
----> 1 RT.InstrumentDevice = 3

/Users/iankatz/.virtualenvs/artoo/sdtkiai/coi-services/extern/pyon/pyon/util/containers.pyc in __setattr__(self, key, value)
     89             raise AttributeError('%s conflicts with builtin.' % key)
     90         if dict.__getattribute__(self, DICT_LOCKING_ATTR):
---> 91             raise AttributeError('Setting %s on a locked DotDict' % key)
     92         if isinstance(value, dict):
     93             self[key] = DotDict(value)

AttributeError: Setting InstrumentDevice on a locked DotDict

><> 

```