In order to avoid computing terms of the Arrhenius function that would
go to 1 (i.e. when either b0=0 or c0=0), we have split the ARR rate law
function into ARR_abc, ARR_ab, and ARR_ac functions.

ARR_abc: Use for nonzero a0, b0, c0
ARR_ab:  Use for nonzero a0, b0 (skips computation of the c0 term)
ARR_ac:  Use for nonzero a0, c0 (skips computation of the b0 term)

We have also updated the models/saprc99.eqn and models/saprcnov.eqn
equation files accordingly.

Signed-off-by: Bob Yantosca <yantosca@seas.harvard.edu>

04_input_for_kpp.rst
- In the #EQUATIONS section:
  - Take equations from the saprc99 mechanism
  - Also add descriptive note block
- In the #EQNTAGS section:
  - Use consistent example in the #EQNTAGS section
- In Auxiliary files for Fortran90 section:
  - Add UserRateLawsInterfaces.f90 to the table

05_output_from_kpp.rst
- Use consistent equation <1> as in 04_input_from_kpp.rst

Signed-off-by: Bob Yantosca <yantosca@seas.harvard.edu>

Now use "GNU General Public License v2.0 or later", which is one of the
accepted license fields to make a Zenodo DOI.  This was preventing
automatic generation of DOIs when new releases are pushed to Github.

Signed-off-by: Bob Yantosca <yantosca@seas.harvard.edu>

Signed-off-by: Bob Yantosca <yantosca@seas.harvard.edu>