start on a common #include <pdal/pdal.hpp> for external  usage #228
remove unnecessary includes
Merge branch 'master' into issues/228-common-include
Merge branch 'master' into issues/228-common-include
only include base stuff in <pdal/pdal.hpp>, not Drivers, Filters, and Kernel, which a user can include separately as needed
Merge pull request #228 from PDAL/issues/228-common-include

common include file for PDAL