Support for lazy_value in parametrize_plus

[smarie]

A missing part for V2 (convergence between case functions and pytest mechanisms, see #90 ) is the capability to not create a fixture for every case function. Indeed this is not necessarily needed: for example when the case function has no parametrization at all and does not depend on any fixture.

Also we might argue that when a case function only has parametrization (case generator) but no fixture dependency (no parametrize plus!) this can also be done without creating a fixture.

The proposed mechanism is

from pytest_cases import parametrize_plus

def valtuple():
    return 1, 2

def val():
    return 2

@parametrize_plus("a,b", [value_ref(valtuple),
                          (1, value_ref(val))])
def test_foo(a, b):
    assert (a, b) == (1, 2)

In terms of implementation, the list passed to parametrize_plus would be a custom list with a hook on __getitem__.