Various Patches for pow(x,i) to powi(x,i), T_p() local vars, and lower p limit notifications

[henningjp]

These commits fix several issues:

• Integer pow(x,i) function introduced in Specialize pow for int. #43 causing errors in some compilers/settings where std::pow(x,y) was required; like in the reverse functions, region 3 Newton-Raphson solver and transport property equations. Renamed optimized pow(x,i) function to powi(x,i) and used throughout forward IF97 function definitions where applicable. Closes Newton-Raphson failing in parts of Region 3 #44
• Possible global variable conflict in p_T(). Closes warning message for "A" variable - global declaration #42
• For functions of (T,P), lower pressure limit not checked; upper limit already checked correctly. Closes out_of_range error not raised #41
• README updated for Mathcad wrapper and newer versions of Visual Studio (CMake -G option).

Testing

Full regression testing performed using Mathcad interface Verification Worksheet.
if97_validation2024.pdf

Tested upper/lower pressure limit checking in all treen IF97 temperature ranges for proper behavior.