added some basic methods to linalg

- Added matrix multiplication, addition, subtraction, scaling and elementwise multiplication to linalg
- Added a new Util module for utility functions and set it to use the same backend as the Core module
- Fixed a small mistake in the unit test for elementwise_square

@lisitsyn, @lambday, @karlnapf Please take a look
could you please add just a line for doxygen here? maybe just @param and @return?