adds flag to allow factorable requirement insertions, default = False

[jaypantone]

Testing plan: after merging the PR to show number of rules produced per strategy, try some runs with and without factorable requirement insertions, to see how much the universe is expanded.

[jaypantone]

Tested for ~10 minutes on Av(2413) with requirement_placements(2), both allowing and disallowing factorable requirement insertion.

Disallowing:

                                                   Number of                                Number of
                                                applications    Time spent    Percentage        rules
        ------------------------------------  --------------  ------------  ------------  -----------
        requirement placement                          4,800       0:03:33           33%       30,084
        add rule                                     176,203       0:02:47           25%            -
        row and column separation                     96,906       0:01:09           10%       31,263
        requirement insertion up to length 2           4,800       0:00:40            6%       31,826
        requirement corroboration                     81,568       0:00:37            5%       15,538

Allowing:

                                                   Number of                                Number of
                                                applications    Time spent    Percentage        rules
        ------------------------------------  --------------  ------------  ------------  -----------
        requirement placement                          4,672       0:03:13           29%       27,552
        add rule                                     176,408       0:02:57           27%            -
        row and column separation                    103,015       0:01:08           10%       30,031
        requirement insertion up to length 2           4,672       0:00:52            8%       38,249
        requirement corroboration                     88,468       0:00:34            5%       14,064

[jaypantone]

Disallowing:

    ClassDB status:
        Total number of combinatorial classes found is 156,209
        is_empty check applied 73903 time. Time spent: 0:00:00
    Queue status (currently on level 5):
        Size       working    current (set 1)    next    underlying    all labels
        -------  ---------  -----------------  ------  ------------  ------------
        Queue 0          0                  0       0             1             1
        Queue 1          0                  0       0             6             8
        Queue 2          0                  0       0            89           102
        Queue 3          0                  0       0           630           766
        Queue 4          0                  0       0          6074          7621
        Queue 5         18                327  76,475        110124        147711

Allowing:

    ClassDB status:
        Total number of combinatorial classes found is 169,175
        is_empty check applied 81170 time. Time spent: 0:00:01
    Queue status (currently on level 5):
        Size       working    current (set 1)    next    underlying    all labels
        -------  ---------  -----------------  ------  ------------  ------------
        Queue 0          0                  0       0             1             1
        Queue 1          0                  0       0             6             8
        Queue 2          0                  0       0            89           102
        Queue 3          0                  0       0           630           766
        Queue 4          0                  0       0          6338          8026
        Queue 5          7                900  82,967        122931        160272

[jaypantone]

We should discuss what the default should be. I suspect that we would not often manually modify packs to change the default, so what we decide the default should be is probably what all runs will be done with.