C API fix: adios_init()

[pnorbert]

Since non-MPI codes don't work with an MPI build of ADIOS, and vice versa, we can unify the adios_init* functions. Let's have adios_init() and adios_init_config() only, which has an extra MPI communicator argument when built with MPI.

This follows how the Fortran API is done.

[williamfgc]

This should be done!