Dense graph solvers are unable to accept upper/lower trianglar matrices.

[shinmorino]

All dense graph solvers are able to deal only with symmetric matrices.
symmetrize() has been added to enable upper/lower triangle matrices, there's a misunderstanding on the equation.

Fix:

1. Check if a given matrix is symmetric.
   1-1. If symmetric, the matrix will be used as is.
2. If asymmetric, check if the matrix is upper/lower triangular.
   2-1 If triangular, symmetrize the mat.
3. Error: Raise exception.

This fix will be applied to all dense graph solvers and functions.

[shinmorino]

Fixes are committed at 45d9005.
Tests have been added.
Py and CPU modules have been tested.

[shinmorino]

This bug is from beta2.
Before beta2, sqaod is assumed to accept only symmetric matrices, and beta2 is also able to handle symmetric matrices.

[shinmorino]

Done. 7145fca.