Add a way to retrieve the computation time of the powerflow

[BDonnot]

It is not documented (my bad...) but the Backend has an attribute called comp_time that can be filled with the actual time spent in the powerflow (this can be used to make benchmark between different backends easier).

Adding a first version of it in the package would not be difficult, it would require to change 2 functions:

    def reset(self,
              path : Union[os.PathLike, str],
              grid_filename : Optional[Union[os.PathLike, str]]=None) -> None:
        logger.info("Reset backend")
        self.load_grid(path, filename=grid_filename)
        self.comp_time = 0.  # added line

and

    def runpf(self, is_dc: bool = False) -> Tuple[bool, Union[Exception, None]]:
        logger.info(f"Running {'DC' if is_dc else 'AC'} powerflow")

        start_time = time.perf_counter()  # changed

        if self._check_isolated_and_disconnected_injections and self._native_backend.check_isolated_and_disconnected_injections():
            converged = False
        else:
            beg_ = time.perf_counter()   # added
            results = self._native_backend.run_pf(is_dc, self._lf_parameters)
            end_ = time.perf_counter()  # added
            self.comp_time += end_ - beg_  # added
            converged = self._is_converged(results[0])

        end_time = time.perf_counter()  # changed
        elapsed_time = (end_time - start_time) * 1000
        logger.info(f"Powerflow ran in {elapsed_time:.2f} ms")

        return converged, None if converged else DivergingPowerflow()

Also as a reference, when benchmarking times in python it's best to use time.perf_counter() rather than time.time()

I can of course do a PR for this if you prefer :-)

Indeed, perf_counter() is guaranteed to be monotonic (always increase) as per the doc https://docs.python.org/3/library/time.html#time.perf_counter

But time.time() can sometimes go backward (https://docs.python.org/3/library/time.html#time.time)

While this function normally returns non-decreasing values, it can return a lower value than a previous call if the system clock has been set back between the two calls.