Updates and corrections to surface tension

[wilfonba]

Description

This PR addresses issues brought up in recent discussions and issues regarding the documentation of surface tension. It also corrects a bug introduced in #688 where the surface_tension logical wasn't being MPI broadcast to all processes. It also adds extra checks to ensure the num_fluids = 2 and patch_icpp(i)%cf_val is specified for each patch. Adding these checks made me realize that the existing surface tension test cases were single fluid and, thus, not testing surface tension. This has also been corrected.

Fixes: #724 #720

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• Something else

Scope

• This PR comprises a set of related changes with a common goal

How Has This Been Tested?

Ran examples/2D_laplace_pressure_jump and observed expected behavior.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 42.98%. Comparing base (a79e2e0) to head (252bfba).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #728      +/-   ##
==========================================
+ Coverage   42.96%   42.98%   +0.01%     
==========================================
  Files          61       61              
  Lines       16314    16314              
  Branches     1882     1880       -2     
==========================================
+ Hits         7010     7013       +3     
+ Misses       8260     8259       -1     
+ Partials     1044     1042       -2     

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