bug1: when writing value characteristic bpdump requires the length before the value
bug2: fixed merging serialized collective metadata index

examples/hello/bpTimeWriter/helloBPWriteStep.cpp

examples/hello/bpWriter/hellBPPutDeferred.cpp

BPWrite_c -> ctypes
XMLConfigTest.TwoIOs -> Test BP Writer 1

Callback Signature classes extend Operator
Operators can be used per Variable, IO or Engine

Continue refactoring IO, ADIOS classes

Changed m_Transports from vector to map, needed by subfile manager

BP3 Deserialization:
Getting subfile seeks and intersection info
Set host language in bindings for row-major and zero-index info

Passed ctest (MPI and nonMPI) excluding ADIOS1 engine, and Python bindings
HDF5, C bindings and C++ tests working
Introduced BeginStep EndStep functions
Renamed IO GetVariable/Attribute to InquireVariable/Attribute

Getting and defining available Variables at Open
Storing Variable position in Metadata, added to VariableBase
mpidummy: Added support for MPI_Reduce, MPI_Gatherv
Added utils and BPLS

Implemented basic buffer reading functionality
Added threading for bp serialization/deserialization
Refactored MPI functionality
Engine is now a reference
Initial Read API

First working examples
To do:
Resolve conflicts
Add tests after merging with current master

Subroutine based, calls the C bindings using opaque pointers
Working helloBPWriter_f in MPI and nonMPI examples
Separated MPI and nonMPI interfaces in Fortran and C
Passing character array by trimming and adding char(0) null terminated

To do:
Add testing

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

Added portable PathSeparator
Fixed format for bp file name

Fixed conflicts for clean compilation
Fixed clang-format
Adding free in C example

Added working example examples/hello/bpWriter/helloBPWriter.c
Changed Engine signature to resolve Write void* at runtime (required by
C bindings).

To do:
CMake infrastructure
Testing

Makefile compiles

To do:
CMake infrastructure
Verify struct adios2_Engine  forward declaration
Add tests

Refactored IO, Variable class due to common functionality