move rmLeadingTrailingWhitespaces to string_utils
string_utils: import a few functions from SusyXSReader

These functions will be used to check the formatting of the xsec files
before passing it on to CrossSectionDB
move isInt from MCWeighter to string_utils
allow to parse additional xsec files

This feature tries to fix #15
This feature is needed when one wants to process a xsec file that is
not in the `SUSYTools/data/` directory. Otherwise, one gets a `-1`
cross section. Details of the implementation:
- implement `susy::utils::filesFromDir` (if one wants to add a
  directory rather than its files one by one)
- implement `isFormattedAsSusyCrossSection`: the SUSYTools
  `CrossSectionDB` does not perform any check on the formatting of the
  xsec files. Since here we will be dealing with private files, we
  need to verify that they are formatted as they should.
- implement `parseAdditionalXsecFile`,
  `parseAdditionalXsecDirectory`. These two functions call
  `CrossSectionDB::loadFile`. However, before doing that they check
  whether the xsec values we are about to read are already stored in
  the existing `CrossSectionDB`, in which case they warn the user that
  they will be overwritten (SUSYTools does not perform such test, it
  just overwrites the elements of the map...).
- add test executable `test_mcWeighter`
drop SleptonXsecReader
forgot to drop SleptonXsecReader from Linkdef (amend 2d7429b)
Merge pull request #16 from gerbaudo/addnl-xsec-files

Addnl xsec files