The vector of length nReactions() holds effective third-body concentrations
for any reaction type.

Enable more generic handling of non-trivial concentration dependencies

* Remove redundante constructors
* Create ArrheniusRate from empty node
* Enable output of C++ type
* Fix pre-exponential factor for stand-alone reaction rates

Declare ReactionRateBase::type as const
Declare (virtual) destructors

Casting to Falloff3 requires specialized cxx_set_object methods.

Only the MultiBulkRate class actually needs to see the methods that
take the shared "DataType" object as an argument and use that to
evaluate the rate.

Further:
* rename get/setData to get/setFalloffCoeffs, and rename associated
  Python properties to 'falloff_coeffs'

…yshev

Also, rename 3-argument versions of setParameters to setRateParameters in
Plog, ArrheniusBase and BlowersMasel, and clarify/streamline some docstrings.

There no longer is a need for Arrhenius3 and BlowersMasel3.

…actor

* Simplify exception handling for Falloff::m_negativeA_ok
* Implement setters/getters for thirdBodyConcentration.

For flame simulations that don't use the 'eField' solution component,
replace the default solution bounds which require this component to
remain exactly zero with arbitrarily wide bounds. Small rounding
errors can occasionally cause the solver to want to have this
component take on a small nonzero value, which could not be satisfied
for any step size, resulting in solver failure.

Previous implementation assumed number format with four letter exponent
blocks; this fix ensures that formatting is correct for very small/large
numbers that use five letter exponent blocks (e.g. 'e+100')