Feature/2p tracer

Continued work from !1520 (closed) here

• renamed pIdx -> phaseIdx

• introduced saturation with enable_if for multiphase tracer (single phase stays untouched)

• use saturation in local residual

• use phaseName for output

• introduced a test case (e.g. 2p lensproblem + tracer in water phase (maybe accumulates at fluid-fluid interface))

Closes #498 (closed)