Package Selection Development, master branch (2017.08.03.)

The MR fixes two issues:

• When configuring a (user) project that didn't have a package_filters.txt file (which most user projects don't), the configuration would continue printing "No package filtering rules read". With this update that line should only be printed once.
• The code now keeps track of which filter rules from the input file were actually used to influence the decision about any package. Since if a rule is not used in the end for anything, that's a strong hint that something went wrong. (Usually somebody making a typo while setting up a sparse build.)

When a problem like that is encountered, the users will get a warning like the following:

...
-- Number of packages configured: 1
CMake Warning at /home/krasznaa/projects/selrule/install/TestExternals/1.0.0/InstallArea/x86_64-slc6-gcc62-opt/cmake/modules/AtlasInternals.cmake:1169 (message):
  Ignored these rule(s) while selecting packages to build:

     + Package1
     - Package3
     - .*

  Make sure that you didn't make a typo, and that there is no duplicate in:

     /home/krasznaa/projects/selrule/test/source/package_filters.txt

Call Stack (most recent call first):
  /home/krasznaa/projects/selrule/install/TestExternals/1.0.0/InstallArea/x86_64-slc6-gcc62-opt/cmake/modules/AtlasFunctions.cmake:423 (atlas_print_unused_package_selection_rules)
  CMakeLists.txt:8 (atlas_project)


-- Generated file: /home/krasznaa/projects/selrule/test/build/x86_64-slc6-gcc62-opt/packages.txt
...

@leggett, hopefully this is what you had in mind as well.