do not GLOB for core pxar files but add them manually -- allows CMake to act on changes

- replaced GLOBing of files (searching for files) in the core lib with listing the actual names -- this way, CMake can act on changes (e.g. additional new files)
- added some comments
log if pixel cannot be correctly decoded

-added log message (level critical) if the pixel cannot be decoded within NUMCOL and NUMROW limits
-(hopefully) helps to debug issue #64
-factorized datatypes into header and impl. files