src: moved 'Complex.is_scalar()' into base
src: changed modifier of 'base::is_scalar' from public to private
src: added 'Complex_fwd.hpp'
src: added 'Matrix_fwd.hpp'
src: modified some code with 'Complex_fwd.hpp' and 'Matrix_fwd.hpp'
src: added instance method 'Matrix.equals()'
test: added 'Matrix.equals()' testing
readme: added 'Matrix.equals()' api
src: added instance method 'Matrix.isApprox()'
test: added 'Matrix.isApprox()' testing
readme: added 'Matrix.isApprox()' api
added source files for linting pass