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`.
Revert "MAINT: add gmock to third-party software."

This reverts commit 96f523314118c9781ae33ea41faa82ab6486024e.
Merge pull request #92 from DO-CV/maint-remove-third-party-gmock

Revert "MAINT: add gmock to third-party software."
Merge remote-tracking branch 'origin' into maint-cleanup-core-test-folder