Surface tension fix for over critical

[eslickj]

Summary/Motivation:

There is an issue with surface tension in that it goes to zero at the critical point and really is undefined beyond the critical temperature. It's only valid in the two phase region, but since it is only a function of temperature, I just calculate the surface tension for whatever the temperature is, and assume the modeler knows what they are doing. To avoid evaluation errors over the critical point, I just use smooth max to make the surface tension approach 0 at near the critical point and above, but I forgot to put the smooth max in the surface tension expression. This is a fix for that.

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[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01 ⚠️

Comparison is base (cc1b0df) 76.83% compared to head (5ee236a) 76.82%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1192      +/-   ##
==========================================
- Coverage   76.83%   76.82%   -0.01%     
==========================================
  Files         390      390              
  Lines       61849    61852       +3     
  Branches    11386    11386              
==========================================
  Hits        47520    47520              
- Misses      11867    11871       +4     
+ Partials     2462     2461       -1     

Impacted Files	Coverage Δ
...al_helmholtz/expressions/surface_tension_type01.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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