Implemented test in respond to #15

[sami6mz]

Added a test to check whether the estimator is robut to (n,1) and (n,) inputs for t and y.

As for now :

• IPW returns aberrant indirect effects if not given .ravel inputs
• g_computation, simulation_based, Gestimator, DML returns ValueError
• multiply_robust is returning an error, but this will be rectified in #35 This is issue #14 .

DML_huber also return an error but it's because dim_x=1, this is already declared in #16.