Use rayon to parallelize the evaluation of the functional - #378

Time spent calculating weighted densities:      0.95 s  ( 9.03 %)
Time spent calculating partial derivatives:     7.07 s  (67.56 %)
Time spent calculating functional derivative:   1.17 s  (11.16 %)
Time spent doing other stuff:                   1.28 s  (12.25 %)
                                              ---------------------
                                               10.47 s

Time spent calculating weighted densities:      1.00 s  (22.04 %)
Time spent calculating partial derivatives:     1.16 s  (25.50 %)
Time spent calculating functional derivative:   1.19 s  (26.28 %)
Time spent doing other stuff:                   1.19 s  (26.18 %)
                                              ---------------------
                                                4.53 s

Time spent calculating weighted densities:      2.96 ms  (11.67 %)
Time spent calculating partial derivatives:    16.60 ms  (65.39 %)
Time spent calculating functional derivative:   2.55 ms  (10.04 %)
Time spent doing other stuff:                   3.27 ms  (12.90 %)
                                              ---------------------
                                               25.38 ms