Create output directory if doesn't exist.

- move code to main.cc source file (used only for Flow123D not for unit
tests)
Remove system/skip_to function.
Create output directory if doesn't exist.

- move code to FilePath class
- create absolute path of output directory
- changes in unit tests
Create output directory of absolute path.

- add test of absolute path under Linux and Cygwin.
- modification of file path unit test.
Optimization of FilePath code.

- split set_io_dirs method
- add method for get absolute path of working directory
Optimization of FilePath unit tests.

- use method for get absolute path in evaluation of checked expression