add get_event_erased to resource class

[artv3]

Since we pass back a erased resource type in LLNL/RAJA#1029, we need to add get_event_erased method class to the resource class.

[trws]

With the suggested change, or an equivalent one, I'm good with this.

[trws]

@artv3: Error from Travis, looks like the ContextInterface doesn't have the method to pass it through:

1: /home/raja/include/camp/resource.hpp:94:50: error: 'class camp::resources::v1::Resource::ContextInterface' has no member named 'get_event_erased'

[artv3]

Thanks @trws, could we sync up after the release to try to fix this? I'm off the next few days but hopefully after we can sort it out.
This PR is needed for the following RAJA branch - LLNL/RAJA#1029.

[trws]

Sure. The problem should be solved by adding a get_event_erased right next to the get_event proxy method here.

[artv3]

Hooray! all test pass!

[trws]

Looks great, thanks!