multi-grid enabled
DONE:
-the grid can host multiple pairs of bra and kets
-little slowdown (16.5->14.1 updates/us) due to vectorization of six variables
-memory save since there are common variables like check or neighbor
-all in the same class (+++for measurements and grid-grid correlations)
TODO:
-test multi-grid to ensure stability
-think about operator(swap) and measurement(corr) concepts
-loop analysis tool would be nice