* Text of nmodl file available if a mechanism.

* MechanismType.code() returns the text of the mod file of the selected mechanism.
If there is no mod file for the implementation, returns "".
Note the first line of the code is the name of the modfile.

* Separated MechanismType file and code (#176)

>>> from neuron import h
>>> s = h.Section(name='s')
>>> mt = h.MechanismType(0)
>>> dir(mt)
['__bool__', '__call__', '__class__', '__delattr__', '__delitem__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__gt__', '__hash__', '__init__', '__iter__', '__le__', '__len__', '__lt__', '__module__', '__ne__', '__new__', '__next__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__setitem__', '__setstate__', '__sizeof__', '__str__', '__subclasshook__', 'action', 'baseattr', 'code', 'count', 'file', 'has_net_event', 'hname', 'hocobjptr', 'internal_type', 'is_artificial', 'is_netcon_target', 'make', 'menu', 'pp_begin', 'pp_next', 'remove', 'same', 'select', 'selected']
>>> mt.select('hh')
0.0
>>> mt.file()
'hh.mod'
>>> print('\n'.join(mt.code().split('\n')[:4]))
TITLE hh.mod   squid sodium, potassium, and leak channels

COMMENT
 This is the original Hodgkin-Huxley treatment for the set of sodium,

* NMODL_TEXT nmodl_filename is the full path from realpath.

* NMODL_TEXT for INCLUDE "fname" comments the INCLUDE line with :::, adds
a ::: commented full path line if realpath available,
inserts the text of the INCLUDE file,
and closes the text with a ::: commented "end INCLUDE fname" line.

* Allow json printer to embed NMODL in the JSON ast

Change-Id: Id3a38a67a104c85b1771638e31d6071e310483c7

* explicit about params in python test

Change-Id: I410cd04f8a0c264638c158333858b95bc949259a