Add libSplash Version Checker

[ax3l]

• read patch level, too
• check for minimum version of libSplash, else ignore as if
  it would be not provided (missing plugins shall not throw
  errors)

[ax3l]

Since the include path changed and the patch level was missing, users with 1.0 will get an error right now instead of a warning.
But this will work fine for later versions of libSplash, throwing something like:

-- Found libSplash: /opt/pkg/filelib/libsplash/1.0 (found version "1.0")
CMake Warning at src/picongpu/CMakeLists.txt:300 (message):
  libSplash version is smaller than the required version 1.1.0 - not using

-- Could NOT find libSplash for hdf5 output

This will later on be transformed in a find_package module, see ComputationalRadiationPhysics/libSplash#78 .