- Reimplement old templates in jinja2 format
- Adapt the generator accordingly
- Merge CMakeLists.txt files (hopefully correctly)

Makes it possible to remove the ROOT dependency from libTestDataModel in
case for e.g. SIO reading.

Defining a function that creates tests and optionally links to the ROOT
I/O if necessary allows for cleaner and definition of individual tests

Making the core podio library independent of SIO (as it has previously
also been done with ROOT). In a similar fashion split off the SIO
dependent parts from the libTestDataModel and try to generate a specific
blocks library that only contains the SIOBlocks stuff. Currently need to
inclue at least one of the SIOBlock headers of the library to actually
force the proper self-registration of the SIOBlocks in the SIOBlock
factory.

Now no longer owns the blocks it reads, but instead creates them for
every event and then passes on the ownership to the EventStore

 - turn on to use boost_filesystem instead of sustem one
 - used only in libSioIO so far

Making the generator accept (and require) these as command line
arguments allows for a bit more flexibility in defining I/O backends.
For backwards compatibility the additional argument to
PODIO_GENERATE_DATAMODEL is currently made optional and defaults to only
the ROOT backend.

Make podio look for a compatible filesystem library itself during the
cmake stage, eventually falling back to boost in case the compiler
doesn't support it natively.

Marking the getEntries and endOfEvent methods virtual and adding them to
the IReader interface allows to re-use the same code for the read tests
independent of the reader that actually provides the data.

NOTE: The current implementation of SIOReader::getEntries is just here
for feature completeness, and should be replaced with a more performant
and const-correct version.

VERBOSE and DEBUG become available as modes to message only in cmake
3.15.

Provide cmake functions to:
- Add the core datamodel library without any I/O backend dependencies.
- Conditionally add a ROOT dict target (if the corresponding code has
  been generated)
- Conditionally add a SioBlocks library target (if the corresponding
  code has been generated).

Example usage in tests/CMakeLists.txt

The usage still requires downstream users to check whether the target
has been generated if they absolutely require it, but at least a lot of
the boilerplate that comes with building (or trying to do so) is now
hidden in these functions.

Co-authored-by: Andre Sailer <andre.philippe.sailer@cern.ch>