Change make plot

### Small changes
* change order in initialisation in ContourConstraint.h
* change GaussData to contain mu, and a vector of sigmas, instead of mu and sigma squared.
* fixed typo in axes ->zaxes in src/Plot.cpp
* rewording of WARNING
* fixed  typos in user/vars_lookups

### Adding managers, Ctypes wrappers (in c++ library and python) 
For each class that needs a manager, the following workflow is applied
* add a manager, that keeps a map of class instances, in particular you can Add.. and Get()
* add extern "C" block that allows access via python ctypes
* incorporate in python routines, by creating "user/ ... .py " list and "add" functions

In this pull request the managers/supported implementations were implemented for
* contours: default, log_x_log_y 
* get_value: gauss constraint and contour constraint

### Make plot routine has switched to using managers instead of json

Plots are now produced according to reference value, instead of hard coded X2 and array entry zero.

Modified user/ files
* vars_lookups and vars_functions are separated from user/axes.py
* contour_constraints and gauss_constraints are separated from user/constraints

added CtypesWrappers
* gauss_constraint, contour_constraint, and contour