[cleanup] fix some typos in documentation strings

I still had some old commits lying around that fixed some typos in comments. Since then I found the codespell program which turned out to be quite helpful in finding common misspellings.

So I guess, I've gathered enough fixes by now to justify a merge request.

Similar merge requests for the rest of the Dune core modules will follow soon.

cmake/modules/DuneCMakePackageConfigHelpers.cmake

@@ -23,7 +23,7 @@ else(CMakePkgConfigHelpersMacroFile)
     if(NOT PMPKG_INSTALL_DESTINATION)
       message(FATAL_ERROR "configure_package_config_file needs an option INSTALL_DESTINATION with a relative path")
     endif(NOT PMPKG_INSTALL_DESTINATION)
-    # asume that PMPKG_INSTALL_DESTINATION is relative
+    # assume that PMPKG_INSTALL_DESTINATION is relative
     string(REGEX REPLACE "[^/]+" ".." prefix_path ${PMPKG_INSTALL_DESTINATION})
     set(PACKAGE_INIT "# Set prefix to source dir
 get_filename_component(PACKAGE_PREFIX_DIR \"\${CMAKE_CURRENT_LIST_DIR}/${prefix_path}\" ABSOLUTE)