[Kinetics] Allow negative pre-exponential factor for falloff reactions

[speth]

If both the high and low rate coefficients for a falloff or chemically activated reaction are negative, then there is no problem evaluating the rate, so it can be allowed in the same cases where a negative rate would be allowed for an elementary reaction, i.e. if the presence of the negative pre-exponential factors has been explicitly declared.

This complements the change made for three-body reactions in #822.

Checklist

• There is a clear use-case for this code change
• The commit message has a short title & references relevant issues
• Build passes (scons build & scons test) and unit tests address code coverage
• The pull request is ready for review

[codecov]

Codecov Report

Merging #868 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #868   +/-   ##
=======================================
  Coverage   71.57%   71.57%           
=======================================
  Files         372      372           
  Lines       44503    44509    +6     
=======================================
+ Hits        31851    31858    +7     
+ Misses      12652    12651    -1     

Impacted Files	Coverage Δ
include/cantera/kinetics/Reaction.h	100.00% <ø> (ø)
src/kinetics/Reaction.cpp	86.43% <100.00%> (+0.28%)	⬆️

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[bryanwweber]

I pushed a small change for the test function to use the return value from the convert() method. Otherwise, this looks good to me.