Implement carried-forward string handling option 4

[delosh653]

Implements option 4 for carried-forward string handling:

Evaluate all carried-forwards based on position in string from i=1:n (n = the maximum length of a CF-string). Evaluate each CF based on the include before and after (if available). At the end of each evaluation for each subject's CF at position i, do not further evaluate strings where the CF at the current position has been marked for exclusion. Whittle down the search space in this manner until either there are no more subjects to evaluate or we have evaluated all subjects.

There also may be some future optimization for this option if we choose to go with it.

Also:

• fixes a small bug in option 3
• Implements option 5: uses the sweeping strategy of option 4, but with the answers from acf-answers evaluated for each carried-forward.

[delosh653]

I'd like to also explicitly add this in to the docs for testacf() -- may be waiting for the checks on #69 to fix.