Allow adjusting of spring exponent

[awvwgk]

$constrain
   DISTANCE: 1, 2, auto, 4
$end

[codecov]

Codecov Report

Merging #357 into master will decrease coverage by 0.00%.
The diff coverage is 20.00%.

@@            Coverage Diff             @@
##           master     #357      +/-   ##
==========================================
- Coverage   40.57%   40.56%   -0.01%     
==========================================
  Files         302      302              
  Lines       50589    50600      +11     
==========================================
  Hits        20527    20527              
- Misses      30062    30073      +11     

Impacted Files	Coverage Δ
src/constrain_param.f90	1.35% <0.00%> (-0.03%)	⬇️
src/constrain_pot.f90	8.09% <0.00%> (ø)
src/scanparam.f90	11.11% <ø> (+0.09%)	⬆️
src/type/setvar.f90	78.57% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 430b391...fe06083. Read the comment docs.