Fix wrong result of ConstitutiveMaterial.plot() if any stretch <= 0

[adtzlr]

this raises an error now to avoid the wrong results.

fixes #852

>>> import felupe as fem
>>> 
>>> umat = fem.LinearElastic(E=1, nu=0.3)
>>> x = fem.math.linsteps([1, 0], num=50)
>>> ax = umat.plot(ux=x, bx=x, ps=x)
...
ValueError: All stretches must greater than 0.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.54%. Comparing base (6704ed0) to head (3ab32a5).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #872   +/-   ##
=======================================
  Coverage   97.54%   97.54%           
=======================================
  Files         149      149           
  Lines        5739     5749   +10     
=======================================
+ Hits         5598     5608   +10     
  Misses        141      141           

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