setAttribute: Explicit C-String Handling

[ax3l]

Explicitly handle C-strings in order to avoid implicit casts. Makes bindings easier.

Split-off from #333

[anokfireball]

Now here's one I don't understand:

The actual unit test with gcc@7.3.0+MPI succeeds, yet the same config (+Python) in the coverage test throws:

Test can only be run with at least two ranks
terminate called after throwing an instance of 'std::runtime_error'
  what():  A Record can not be written without any contained RecordComponents: position
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
ParallelIOTests is a Catch v2.2.1 host application.
Run with -? for options
-------------------------------------------------------------------------------
hdf5_write_test_zero_extent
-------------------------------------------------------------------------------
/home/travis/build/openPMD/openPMD-api/test/ParallelIOTest.cpp:146
...............................................................................
/home/travis/build/openPMD/openPMD-api/test/ParallelIOTest.cpp:146:FAILED:
due to a fatal error condition:
  SIGABRT - Abort (abnormal termination) signal
===============================================================================
test cases:  3 | 2 passed |1 failed
assertions: 13 |12 passed |1 failed

UPDATE
This also happens on dev, so it's not a blocker for this PR.