Adding ability to delete variables
Users can force package download

CMake may find a package that does not work with the current compiler/setup.
It is possible for the user to force the download of a package from the
command-line
Merge branch 'refactor' of github.com:UCL/GreatCMakeCookOff into refactor
refactoring bits in package lookup
package_FOUND variable was being deleted right after being set
:lipstick: Fit into 80 columns
Can now pass list via passon_variables

List where being concatenated into a single string item.