Fix MSVC compilation issues on Windows (M_PI, typeof)

[pansuestc]

This pull request addresses two compilation errors encountered when building the project on Windows using the MSVC compiler.

1. M_PI Undeclared Identifier

   • Error:

     D:\code\bembel\Bembel\src\util\Constants.hpp(42): error C2065: “M_PI”: undeclared identifier
     D:\code\bembel\Bembel\src\util\Constants.hpp(42): error C2737: “Bembel::Constants::mu0”: must initialize constexpr object
     

   • Problem: On some platforms and compilers (including MSVC by default), M_PI is not automatically defined in <cmath> or <math.h>. It often requires a specific macro like _USE_MATH_DEFINES to be set before including these headers.
   • Solution: Added an #ifndef M_PI guard with a standard definition of PI in src/util/Constants.hpp. This ensures M_PI is available even if the compiler settings don't provide it.

     #ifndef M_PI
     #define M_PI 3.14159265358979323846
     #endif
     
     // Original code that uses M_PI
     constexpr double mu0 = 4 * M_PI * 1e-7;

2. typeof Identifier Not Found

   • Error:

     D:\code\bembel\examples\LaplaceAdjointDoubleLayerH2.cpp(93): error C3861: “typeof”: identifier not found
     D:\code\bembel\examples\LaplaceAdjointDoubleLayerH2.cpp(93): error C2923: "Eigen::GMRES": "typeof" is not a valid template type argument for parameter "_MatrixType"
     

   • Problem: typeof is a non-standard language extension, typically supported by GCC and Clang. It is not a standard C++ keyword and is not recognized by MSVC.
   • Solution: Replaced the non-standard typeof with the standard C++11 keyword decltype in examples/LaplaceAdjointDoubleLayerH2.cpp. decltype(expr) is the standard way to obtain the type of an expression.

     // Before: non-standard typeof
     GMRES<typeof(system_matrix), IdentityPreconditioner> gmres;
     
     // After: standard C++11 decltype
     GMRES<decltype(system_matrix), IdentityPreconditioner> gmres;

Verification:

These changes have been tested and successfully resolve the compilation errors on Windows using Visual Studio 2022 (MSVC).