Allow default paths for opencascade detection

OPENCASCADE_DIR is just a hint and might be invalid.

(In FindTRILINOS.cmake the full path is known due to the inclusion of the
Trilinos project configuration. Thus, those options ensure that no wrong
library is accidentally picked up.)
Merge pull request #244 from tamiko/fix_opencascade_detection

Allow default paths for opencascade detection
added read support for step files, along with its tests. write support is a little bit more complicated, we'll work on it
added write support (with tests) for step files
added namespace and () to the read_STEP and write_STEP name functions in the doc/changes.h file; removed some extra empty line from the function implementation in utilities.cc
Merge pull request #245 from andreamola/added-STEP-support

Added step support