add functions to trim strings

[dhebbeker]

This code has originally been written for #82. But before that has been merged, the need for the trim functions became obsolete due to changes in the code of #82. Thus the commits which were originally part of #82 have been rebased. This way these functions can be added later if necessary.

• make sure this function is actually necessary
• ensure sufficient code coverage

[dhebbeker]

I don't see any need for this anytime soon.