Add tests folder in project containing all kind of tests

Closes #186 (closed)

• Add a new tests folder on project level with subfolders:
  • unit-tests
  • regression-tests
  • numerical-tests
• Moves all tests to the corresponding sub folder

changes in cmake:

• easier finding of files as unit tests are now not located in src/ anymore
• building source groups for visualstudio with cmake TREE command.
• moves /wd4458 on project level, as declaration of 'XXX' hides class member is used everywhere.

TODO:

• check if source groups works in Visual Studio