Inconsistent capitalization in single letter variables

[white238]

I noticed some inconsistent capitalization (U vs u, dU_dt vs du_dt) in a function. I don't know how pervasive this is but we should at least be consistent.

Example:

https://github.com/LLNL/serac/blob/07b8ef1b3658bee2b66b78fe900595e3408e31ac/src/serac/numerics/odes.cpp#L148-L183

[samuelpmishLLNL]

This discrepancy was intentional (although poorly explained in the implementation) to be consistent with a theory reference document that uses lower case "u" to denote the vector of unconstrained components, and upper case "U" for the constrained components, I think.

We could amend the comment here to include this information:

   // evaluate the constraint functions at a 3-point 
   // stencil of times centered on the time of interest 
   // in order to compute finite-difference approximations 
   // to the time derivatives that appear in the residual 
   //
   // Note: variables with an upper-case "U" represent
   // the constrained / prescribed values
   U_minus_ = 0.0; 
   U_       = 0.0; 
   U_plus_  = 0.0; 

It could also be put on the doxygen explanation of the members. It looks like Josh did a very thorough documentation pass on this class initially, but some of the descriptions are inaccurate (my fault for not looking close enough on the code review).