The `PrintMatrix` was declared in the global namespace which means that
if you define your own PrintMatrix macro you can have a name clash.

This PR "qualifies" the name using the prefix TMVA_DNN_

In MethodDNN one could start training with a batch size larger than the
number of events in training leading to non-sensical output. TMVA now
properly warns (fatal warning) and suggests a fix (decrease batch size).

Explicitly use bools in if's
Use `.empty()` instead of `.size() == 0`

Simple to //-ise over number of output classes. Doing the same over
number of trees seems to yield no benefit for ~1000 depth=3 tree so this
was skipped (binary and multi class).

Optimal would be to //-ise over events, but the current design makes
this tricky.

The INCLUDE_DIRECTORIES property only became available in
CMake 3.11, so we have to use COMPILE_FLAGS instead, which
is already available in our required version (CMake 3.4.3).

Note: The ROOT_ARCHITECTURE value of win32old is not found anywhere
else in the code, and the files referred to don't exist anymore, so
the obsolete linkdef file has been removed too.