Add get_simulator_index to execution

[markaren]

Allow retrieving the simulation_index using a simulator name.
This is simply just a useful function to have.

The implementation and function name in this PR must be rewritten once we decide to go forward with #384, but lets just add this to our API now.

[eidekrist]

What is the use case for this? With an execution you always have a mapping between simulator name and index from either:

• simulator_index execution::add_slave()
• std::pair<execution, simulator_map> load_ssp()
• std::pair<execution, simulator_map> load_cse_config()

[markaren]

The use case was originally #426, but it was implemented in another way.

simulator_index execution::add_slave()
std::pair<execution, simulator_map> load_ssp()
std::pair<execution, simulator_map> load_cse_config()

You might find yourself in a situation where you don't have immidate access to these. Also I have never liked the std::pair<execution, simulator_map> way.

I'm fine with this PR not being merged though.

[kyllingstad]

As a general rule, I think that any PR that adds functionality should be justified with a concrete use case, or there must be some argument for why it is a superior solution (e.g. it removes a known source of error in client code).

This is simply just a useful function to have.

YAGNI. ;)

[markaren]

Closed based on feedback. May revisit in the future.