add simple RMGIpc for remage-cpp -> python intercom

[ManuelHu]

this is very much WIP and needs good testing before releasing

this is a simple IPC mechanism. Its main features:

• one-way communication of unicode strings from the C++ side to python
• asynchronous messages do not get an acknowledgement back
  (this is meant for informational messages that the python code needs after running remage-cpp, e.g. output file post-processing).
• synchronous messages require an acknowledgement signal from python. for this, the POSIX signal SIGUSR2 is used
  (this is meant for actions that python code should perform synchronously, e.g. working on input files)

by using this mechanism we can avoid re-implementing ugly logic in the python wrapper (e.g. guessing output file names, etc.) and get that data just form the C++ application.

this PR does not implement any actions on the python side yet. But with minor additions, this could be used for #190

• the biggest changes will be on the pydataobj side, as there is no way to call the functionality of lh5concat from python code...
• also, as @tdixon97 noted: lh5concat does currently load all files into memory, and does not use an iterator with smaller buffer size. This might not be what we want

[tdixon97]

@ManuelHu I guess it wouldnt be a big modification to make lh5concat iterate? Seems worthwhile also for other applications

[gipert]

I think lh5concat changes shouldn't be hard to implement

[ManuelHu]

this should now be mostly ready. I would like to implement any actual merging in a follow-up PR to keep apart the infrastructure work and the actual features.

[tdixon97]

I don't understand what it is, but looks good