Add checks for passing explicit t-gradient or Jacobian to stiff solver

[ScottPJones]

Cleaned up the way of checking both keyword arguments and t-gradient & Jacobian

[coveralls]

Coverage remained the same at 85.096% when pulling 398b236 on ScottPJones:spj/updatechk into 48b6fd9 on rveltz:master.

[coveralls]

Coverage remained the same at 85.096% when pulling 398b236 on ScottPJones:spj/updatechk into 48b6fd9 on rveltz:master.

[codecov-io]

Codecov Report

Merging #33 into master will not change coverage.
The diff coverage is 55.55%.

@@           Coverage Diff           @@
##           master      #33   +/-   ##
=======================================
  Coverage   85.09%   85.09%           
=======================================
  Files           4        4           
  Lines         208      208           
=======================================
  Hits          177      177           
  Misses         31       31

Impacted Files	Coverage Δ
src/common.jl	85.59% <55.55%> (ø)	⬆️

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[coveralls]

Coverage remained the same at 85.096% when pulling 2f037a1 on ScottPJones:spj/updatechk into 48b6fd9 on rveltz:master.

[coveralls]

Coverage remained the same at 85.096% when pulling 2f037a1 on ScottPJones:spj/updatechk into 48b6fd9 on rveltz:master.