Prepare version 2.3.1

[jcbf]

bump to version 2.3.1

[coveralls]

Coverage remained the same at 78.672% when pulling 31f64d0 on jcbf/release-2.3.1 into 8dfd517 on master.

[codecov]

Codecov Report

Merging #35 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #35   +/-   ##
=======================================
  Coverage   78.61%   78.61%           
=======================================
  Files           1        1           
  Lines         491      491           
  Branches      166      166           
=======================================
  Hits          386      386           
  Misses        105      105

Impacted Files	Coverage Δ
smf-spf.c	78.61% <ø> (ø)	⬆️

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