`remage_run` return table in memory

[tdixon97]

In a similar vain to #600 it might be nice if remage_run could return the output file in memory if no output file in requested.

This again would be nice for quick tests without ever writing files to disk

[ManuelHu]

But hoe should that work? The c++ app will always need to write to a file to read the data back in on the python side?

[tdixon97]

Yeah I guess the only way is basically keep writing the tmp files, but read them in in the python wrapper.
Not a huge benefit in the end, but might be nice for very very new users

[ManuelHu]

wouldn't this be extremely constrained? Most of the post-processing tooling uses the LH5Iterator in some place, which would not work with the in-memory tables?