Updating cobra version requires manual deletion of build artifacts

[Ronoman]

When changing the REV field in CMakeLists:14 and running colcon build --packageg-up-to cobra_vendor, if you have previously built a different version of cobra, the following error is returned:

<other packages building>...

Starting >>> cobra_vendor
--- stderr: cobra_vendor                                
fatal: bad object aa397fb86a91cd320def1f6d126c55290222b582
From https://github.com/nimble-code/cobra
   a0990d4..aa397fb  master     -> origin/master
error: Your local changes to the following files would be overwritten by checkout:
        bin_linux/cobra
        bin_linux/cwe
        bin_linux/find_taint
        bin_linux/icobra
        bin_linux/scope_check
Please commit your changes or stash them before you switch branches.
Aborting
CMake Error at /home/spaceros-user/src/spaceros/build/cobra_vendor/cobra-4.1/tmp/cobra-4.1-gitupdate.cmake:144 (message):
  Failed to checkout tag: 'aa397fb86a91cd320def1f6d126c55290222b582'


make[2]: *** [CMakeFiles/cobra-4.1.dir/build.make:96: cobra-4.1/src/cobra-4.1-stamp/cobra-4.1-update] Error 1
make[1]: *** [CMakeFiles/Makefile2:78: CMakeFiles/cobra-4.1.dir/all] Error 2
make: *** [Makefile:129: all] Error 2
---
Failed   <<< cobra_vendor [2.08s, exited with code 2]

I'm unsure how to do a force checkout in ExternalProject_Add, Google is failing me.

[Ronoman]

Possible solution would be to update the VER flag, so that ExternalProject_Add checks out the project into a new directory. Seems like a bit of a hack instead of a true fix. @mjeronimo thoughts?

[mjeronimo]

@Ronoman After changing the REV field, I would just remove the spaceros/build/cobra_vendor directory to get rid of previous build artifacts. Not the most elegant solution, but there isn't a "colcon clean".

FYI - I found a discussion on the 'clean' topic: colcon/colcon-core#37