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.

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

signed char needed by int8_t in C bindings
Added tests

Using language primitives in test data instead of fixed width integers.
casting size_t to unsigned long in mpidummy to remove warning on Mac