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.
Fix wrong scaling in transport.cc

- fix test 14 to cover this case
Merge branch '1.8.0_master' of github.com:flow123d/flow123d into 1.8.0_master
Fix unit_test/fields/field_set
Fix conversion between string and wstring in python loader caused by differences between python 2 and 3.
Test 15 for implicit transport.
- required to fix detection of changed velocity in assembling the transport matrices and vectors
Fixes 187.
Carefull tast of FlagArray::match
Merge branch '1.8.0_master' of github.com:flow123d/flow123d into 1.8.0_master