Add symmetric member to linear operators

[mrava87]

• Add a boolean member called symmetric that defines is the operator is symmetric or not. This will be always False if the operator is not square, while it can be fixed for a square operator or depend to its input parameters like in the case of Block and BlockDiagonal operators
• Add also a 'validation' method to LinearOperator called isymmetric which validates the following equality <x, Ay>==<y, Ax>
• Check symmetric when solving an inverse problem in one of the classes that invoke script solvers and use it to choose the most appropriate solver (e.g., cg for symmetric operators)

[rohanbabbar04]

If this issue is still open, would like to work on it..

[mrava87]

Yes, this is open :) I would suggest to start looking into it but wait to work actively as most of the changes will be done in the linearoperator.py file so it is wise to wait until we approve this PR #482

[cako]

Maybe a good time to get back to this