Fix target initialization issue in implicit time drivers

Description

Fixes a bug reported in #194 (closed).

Note: This revision only addresses the case of target values set as initial conditions in AbstractTimeDriver.set_scenario(init=...). In other words, the case where the right-hand sides of target equations target_name == rhs_value are dynamically evaluated in time is not supported.

Code changes

• Activate targets in both the initial and time-dependent problems;
• New method ImplicitTimeDriver._solve_intrinsic_problem to suppress duplicated code.

Closes #194 (closed)