Upgrade to version 0.6.0

Breaking changes

change the interface of the newton_pf function to reflect pandapower change in their latest version (arguments ref has been added). You can still use the old newton_pf function, with the old signature by importing newtonpf_old instead.
SecurityAnalysis now also returns the active flows when calling security_analysis.get_flows()
change the file names (python side) to be compliant with pep 8. You can no longer do things like from lightsim2grid.LightSimBackend import LightSimBackend change it to from lightsim2grid import LightSimBackend (preferred method)
change the file names (python side) to be compliant with pep 8. You can no longer do things like from lightsim2grid.initGridModel import init change it to from lightsim2grid.gridmodel import init (preferred method) (same for GridModel class)

a bug that lead to the wrong computation of the dc powerflow in case of sn_mva != 1. and phase shifters.
bug preventing to use the NICSLU linear solver in the GridModel
compilation warnings on clang (missing virtual destructor, unused variables, etc.)
a bug in the SecurityAnalysisCPP: when it diverges for some contingencies, the others were not simulated properly.
LightSimBackend now contains members for shunts and ***_theta as it does for the other quantities. This improves the consistency, but most importantly fixes some bugs when used in earlier grid2op versions

possibility to compute the active flows using the BaseMultiplePower
possibility to change linear solver used when performing a DC solver
possibility to make powerflow with distributed slack bus (only for newton raphson at the moment)
access (read only) to the element of a lightsim2grid grid with the get_XXX (eg get_loads()) methods (see documentation)
direct access to the solver used in the grid model python side
unittest in circleci.
[all kind of solvers based on different linear solvers (Eigen sparse LU, KLU or NICSLU) for Newton Raphson and DC approximation (9 solvers in total)

use of steady_clock to retrieve the ellapse time c++ side
refactoring of the c++ part to use template mecanism instead of inheritance for the Newton Raphson and DC solvers.
GridModel now contains two different solvers, one for AC powerflow and one for DC powerflow.
error message in the solver are now embedded in an Enum instead of being integers, for better readibility.
error message when the powerflow diverge (error are read from c++ now)