C: Make C bindings always on and embedded into libadios2

Misc test improvements

changed ndim usage for apple compiler

Consolidate cmake deps

Fixes #222

Testing: Make ADIOS1WriteRead test run in parallel

Add plugin engine

TO achieve this goal, GoogleTest.cmake is extended to allow
exec_wrapper and SmallTestData.h now would generate data based on
step&world_rank&world_size.
ADIOS2ADIOS1WriteADIOS1Read2D1x8/2x4/4x2 now runs in MPI.

Fix misc mac issues

Removed iostream header, not used

Fixing a bug in BP1Base root name

Initial C bindings

Added GetVariableBase* in IO needed by C bindings and real-time
applications, nullptr is a valid state in return.

Added c bindings test under testing/adios2/bindings/c
Updating ReadMe.md to include ADIOS2_USE_C for C bindings

Builds libadios2_c.a and libadios2_c.so 
Verified with and without MPI
Added nonMPI hello bp write example
Added new ADIOS2_USE_C ON/OFF cmake argument

To do:
Add C tests and cmake support for the latter

signed char needed by int8_t in C bindings
Added tests

Fixing issue #230 a bug for path in file name