Changed IdentifierInterface and SetInterface to conform to strange
requirements.
Code now working with new design. Validation is done in inputToSet()
Cleaned up functions. Added some comment.s
modified identifier javadoc
Merge branch 'master' of https://github.com/sirmc/voortgezetprogrammeren
Refactored and renamed several methods to make code cleaner.
Fixed bug in openingCharacterValidation, where it would read a character
even if scanner was empty.
Using nextCharIsWhitespace insetad of nextCharIs(scanner, ' ').
Added check for empty input.

If input is empty, the user will be asked for new input.
Changed some wording in ADTs to be more precise.
Merge branch 'charStackMess' of https://github.com/sirmc/voortgezetprogrammeren
Initialized assignment 2.

Works with input from console.

TODO: Write ADTs
      Input from file
Merge branch 'assignment2' of https://github.com/sirmc/voortgezetprogrammeren
added better parameter names and edited some classes for better readability