Add a virtual destructor to the Solver base class, to allow
for correct polymorphic deletion.

Fixes "-Wdelete-non-virtual-dtor" warning.
Merge pull request #8 from mmassing/fix_missing_virtual_dtor

Add a virtual destructor to the Solver base class, to allow for correct polymorphic deletion.
silence uninitialized-warning
fix? interior filter (when there is no adjacent linesite)
start on non-trivial offset loop sorting
improve cmakelists by making python-module optional, and by improving how python-libraries are detected (no more use of "sed")
make tests and python-module building optional
fix minor issues that give warnings on OSX
small cmake fixes
if we link the main library with libqd we do not need to link later
remove findqd, no longer needed
tweaks to example cmakelists
simplify the cmakelists for the tests (by finding libopenvoronoi in a simpler way, and not linking with libqd)
guard against non-existing input file
cleanup cpp_examples cmakelists (mostly remove finding and linking of libqd)
experiment with using clang
more clang tweaks
minor
debug/build info
new options for building/configuring c++ and python tests.