Ecl sum report step compare

This PR implements functions ecl_sum_report_step_equal() and ecl_sum_report_step_compatible() which can be used to check the report_step <-> true simulation time mapping.

When running ERT with a refcase it is essential that this mapping is the same for the refcase as for the simulations from the forward model. A quite common source of error is that the Schedule file is modified during the modelling process, and the refcase goes out of sync. These functions can be used to detect this situaton.