Extracting the helper funktions into their own helper file

src/helper_files/helper.cpp

src/helper_files/helper.hpp

src/methods/bayesian_linear_regression/Makefile

...bayesian_linear_regression/bayesian_linear_regression.cpp

.../bayesian_linear_regression/bayesian_linear_regression.pl

src/methods/linear_regression/Makefile

src/methods/linear_regression/linear_regression.cpp

src/methods/new_method/Makefile

src/methods/new_method/new_method.cpp