Untemplatize `FindDO.cmake`.

The template file `FindDO.cmake.in` makes library maintenance very tedious.
Store DO-CV version value in a separate file.
Gitignore `cmake/DOVersion.cmake`.
Merge pull request #89 from DO-CV/maint-simplify-maintenance-of-find-do-cmake

[MAINT] Simplify maintenance of `FindDO.cmake`.