0027342: STEP - support C++ streams for import / export

0027342: STEP - support C++ streams for import / export

- STEP low-level parser is converted to C++; required minimal version of flex is elevated to 2.5.37.
- Added possibility to import STEP from stream, see new method XSControl_Reader::ReadStream() (now implemented in STEP only).
- Parsers ported to win_flex_bison 2.5.23 (flex 2.6.4, bison 3.7.1)
- Added support of C++ flex and bison scanners in in CMake scripts
- Some code clean-up in StepFile and around (unused files and functions are eliminated)
- Option to read from stream is added in DRAW command testreadstep for testing ReadStream() function
- Added test bugs step bug27342