NEURON code generation

[iomaganaris]

This issue is created to track the progress of the code generation for then NEURON simulator in NMODL.

After some internal discussion we concluded that we should start with:

1. Create a new protected branch which has slim CI related to NEURON to push small changes there related to the NEURON code generation. This way we don't have to wait for the full CIs of the master branch to push small changes and unblock people collaborating in this project. Intention of this branch is to be merged to master frequently and to be rebased on master as well frequently. (Create protected branch with minimal CI for NEURON code generation #1108)
2. Add hook in nrnivmodl of NEURON to enable then use of an external nmodl binary to generate the code for the selected mechanisms. The rest of the build process in NEURON should stay the same with this. The idea of this is change is to be used by pip install neuron in the CI to accelerate the building of the NEURON version which is installed to be used with NMODL. (Add hook for external nmodl executable in nrnivmodl neuronsimulator/nrn#2625)
3. After being able to generate a C++ file which can be compiled and loaded from NEURON special in Fix mechanism registration function for non POINT_PROCESSes #1111 we can start implementing the actual functions. Idea is to start with implementing the major functionality in NMODL for the fixed time step method which is very similar to CoreNEURON:
   a. https://github.com/neuronsimulator/nrn/blob/master/src/nrnoc/hh.mod to implement nrn_init, nrn_cur, nrn_state and _hh_reg functions which cover all the needed aspects of the density channels
   b. https://github.com/neuronsimulator/nrn/blob/master/src/nrnoc/expsyn.mod to handle the net_receive block
   c. https://github.com/neuronsimulator/nrn/blob/master/src/nrnoc/netstim.mod for handling stimulus
   based on those files we can also create needed integration tests
4. Implement rest of more complicated NMODL constructs for fixed time step method (adding integration tests for those in the meantime)
5. Implement variable timestep method and other NEURON particular functionality

Above is mainly based on inputs from @pramodk , @ohm314 , @1uc and myself.

Here is a table with all the NMODL construct and the related info for each one ( 🚧 WIP 🚧 )

NMODL DSL construct	Functionality	Related MOD file	Issue	PR	Docs	Comments	Progress
Blocks	-	-	-	-	-	-	-
DERIVATIVE	Simple ODE solved in nrn_state()	hh.mod			link	Done	🟢
ASSIGNED	RANGE variables and arrays	hh.mod		#1126	link	Done	🟢
PARAMETER	Read and set PARAMETER variables from interpreter	hh.mod		#1106	link	Under review	🔜
STATE	Make sure that state variables can be used in nrn_state()	hh.mod		#1119	link	Done	🟢
FUNCTION	Functions work in HOC and Python	hh.mod		#1141	link	WIP
PROCEDURE	Procedures work in HOC and Python	hh.mod		#1141	link	WIP
SOLVE	Proper solver is used and simple nrn_state() works	hh.mod		#1119	link	Done	🟢
BREAKPOINT	Generate nrn_cur()	hh.mod			link
INITIAL	RANGE and ARRAY variables are correctly initialised in finitialize()				link
INITIAL	STATE variables are correctly initialised in finitialize()				link
INITIAL	ION variables are correctly initialised in finitialize()				link
LINEAR
NONLINEAR
NET_RECEIVE
KINETIC
UNITS
NEURON
VERBATIM
CONSTANT
BEFORE
AFTER
STEP
DISCRETE
FUNCTION_TABLE
CONSTRUCTOR
DESTRUCTOR
INDEPENDENT
Control Flow	-	-	-	-	-	-	-
WHILE
IF
ELSE IF
ELSE
Other	-	-	-	-	-	-	-
RANGE	RANGE variable registration	hh.mod		#1111	link	Done	🟢
RANGE	RANGE variable functionality	hh.mod		#1119	link	Done	🟢
GLOBAL	Make sure that GLOBAL variables are registered, instantiated and used correctly				link
THREADSAFE	Handle properly GLOBAL variables	hh.mod			link
USEION	ION variables registration and usage	hh.mod			link
READ	ION variables read in nrn_cur()	hh.mod			link
WRITE	ION variables set in nrn_cur()	hh.mod			link
REPRESENTS	Probably do nothing in codegen about that. Just add it in a test	hh.mod			link
NONSPECIFIC_CURRENT	Proper use in nrn_state()/nrn_cur()	hh.mod		#1185	link
SUFFIX	Handle nothing as SUFFIX				link
POINT_PROCESS	POINT_PROCESS location works	expsyn.mod		#1106	link	PR under review	🔜
~
->
FOR_NETCONS
LOCAL
TITLE
DEFINE
INCLUDE
SWEEP
CONDUCTANCE
PROTECT
FROM
WATCH
MUTEXLOCK
MUTEXUNLOCK
CONSERVE
COMPARTMENT
LONGITUDINAL_DIFFUSION
LAG
TABLE
VALENCE
ELECTRODE_CURRENT
POINTER
BBCOREPOINTER
EXTERNAL
COMMENT
SOLVE METHODs	-	-	-	-	-	-	-
cnexp	Solve simpler ODE	hh.mod			#1119	Done	🟢
euler
derivimplicit
sparse
runge
after_cvode
simeq
cvode_t
cvode_v