Add MpiHandshake to enable XGC-COUPLER-GENE communication pattern

bindings: Convert Fortran MPI- and non-MPI implementations to submodules

Convert the separate modules implementing Fortran `adios2_init` and
`adios2_open` generic procedure signatures for MPI and non-MPI variants
into submodules.  This allows the primary modules to be built first and
the implementation submodules built later in separate libraries.

In order to support compilers that do not support Fortran submodules,
use preprocessor conditions to optionally build without submodules as
before.

This will be useful later to add preprocessor conditions to the
implementation sources.

Readme: Conda Badge

Add a badge to install ADIOS2 via conda-forge.
By default, conda-forge packages install without MPI. The user can
control variants via:

- `conda install -c conda-forge adios2` default, nompi variant
- `conda install -c conda-forge adios2=*=mpi*` any mpi-compatible implementation
- `conda install -c conda-forge adios2=*=mpi_mpich*` pick mpich variant.
  Same result as `conda install -c conda-forge mpich adios2=*=mpi*` .
- `conda install -c conda-forge adios2=1.13=nompi*` explicitly forbid MPI.
  Usually not necessary, as the build number offset causes `nompi` to be preferred unless MPI is explicitly requested.

Move async file open functionality to transports

Inline engine fixes

Prevent hang when throwing out of adios constructor in MPI.

docs: Fix typos

Fix adios_reorganize to handle cases where variables are not always p…

docs: clarify documentation for dependency searching

helper: Do not always duplicate MPI_Comm on Comm construction