Deprecate magic numbers for RxnRates.h and Falloff.h

[ischoegl]

Changes proposed in this pull request:

• Deprecate unused magic number int type() methods in RxnRates.h.
• Deprecate magic numbers int getType() and introduce string type() in Falloff.h

I am leaving the remaining magic numbers for kinetics objects as they are, as it may make sense to convert handling of various types to factories instead of having things hard-coded (this is more elaborate and should be handled by a separate PR).

[codecov]

Codecov Report

Merging #733 into master will decrease coverage by 0.03%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #733      +/-   ##
==========================================
- Coverage   70.86%   70.82%   -0.04%     
==========================================
  Files         374      374              
  Lines       43683    43703      +20     
==========================================
  Hits        30954    30954              
- Misses      12729    12749      +20

Impacted Files	Coverage Δ
include/cantera/kinetics/FalloffFactory.h	100% <ø> (ø)	⬆️
include/cantera/kinetics/RxnRates.h	92.78% <ø> (ø)	⬆️
src/kinetics/Reaction.cpp	87.58% <100%> (-0.05%)	⬇️
test/kinetics/kineticsFromScratch.cpp	100% <100%> (ø)	⬆️
include/cantera/kinetics/Falloff.h	52.5% <33.33%> (-15.36%)	⬇️
src/kinetics/FalloffFactory.cpp	54.54% <81.81%> (-45.46%)	⬇️

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[speth]

I'm always glad to see magic numbers go away, and this looks pretty good to me. I had just a couple of minor comments / questions.

[ischoegl]

I switched the type string to "Lindemann" to make things clear for the simple falloff type, and also added some rudimentary tests for the falloff reaction type to the suite.