Include multiple-precision floating-point library MPFR to HyTeG

MPFR is a C library for simulating arbitrary whole number precision for floating-point variables, hence enabling multiple-precision floating-point computations with correct rounding. The precision requested by the user is the number of bits used to represent the mantissa. For more information about MPFR compare: mpfr.org

For better usage in HyTeG, two external C++ wrapper libraries are included as well to enable e.g. operator overloading, automatic memory management, ... One of these is using compile time definition of the variable precision. Compare the distributor website: mpfr::real<> The other one uses a runtime definition of the variable precision. Compare the distributor website: mpfr::mpreal