Moise/non sym toeplitz

[MoiseRousseau]

This PR proposes an implementation of the non-symmetric Toeplitz linear operator (https://github.com/cornellius-gp/linear_operator/issues/76). It extends the actual ToeplitzLinearOperator with an additional constructor argument "row". If not supplied, the Toeplitz operation is still symmetric and the actual behavior is preserved.

• Adding support for non-symmetric Toeplitz operators
• Add tests for non-symmetric Toeplitz operators
• Add an optimized solve routine for symmetric and non-symmetric Toeplitz operator
• All tests pass
• Have the skeleton for optimized Toeplitz inverse

Let me know what you think!