Remove broken support for external metslib.

[larshg]

Fixes #4744
Fixes #605
Fixes #4792

[mvieth]

My assessment of the situation:
If we wanted to support external/system metslib, we would still have to keep the metslib shipped with PCL, because not many package managers offer it: https://repology.org/project/metslib/information (vcpkg, homebrew, and apt for Debian/Ubuntu are missing for example).
We would have to make sure (in PCLConfig.cmake/hv_go.h) that downstream projects find the metslib includes, either the ones from PCL or the external/system installed.
We would also have to adapt the code in hv_go.h and hv_go.hpp to work with both the metslib shipped with PCL, and versions of metslib that may be externally available (e.g. from metslib's latest master branch). This would concern at least the call to setApplyAndEvaluate in hv_go.hpp.
In fact, the simulated annealing in the metslib in the PCL repo has been modified, such that the movit requires additional member functions (apply_and_evaluate, evaluate, apply, unapply). I am not sure what effects these modifications have. I think there is not really a guarantee that GlobalHypothesesVerification works correctly with any metslib except the one in the PCL repo. There is currently no unit test for GlobalHypothesesVerification.
PCL uses the BSD license, as does the metslib shipped with PCL ( e4d6501 ). Normally however, metslib uses a GPL or CPL license. I am no expert in licensing, but it sounds like this could be a problem if someone relies on PCL's BSD licensing.
I am okay with removing the (broken) support for external metslib, but we should probably wrap the metslib code in an additional pcl namespace, to avoid conflicts if a system metslib is available and used (then merge after PCL 1.14.1 is released because of possible ABI breaks).

[larshg]

I tried installing metslib from source and build recognition, but it is incompatible with more than just the setApplyAndEvaluate, ie there are errors with the following:

https://github.com/coin-or/metslib/blob/956478a9c9f1aee01097ff000c6cd13d9f9cc81a/metslib/simulated-annealing.hh#L136-L145

fx. std::variate_generator never made it to the standard afaik. So it seems to me that it would require updates to support the "system" installed version.

Effectively the system used version of metslib haven't been available since 2015, when 4fdd12b was commited. So I don't think many use it - and the few who would try, would just end up with compilations errors.

Regarding licensing, I think it should be up to the user if they install metslib from source, they should already have to conform to their license and since PCL is lesser restrictive, its fine.

[jiapei100]

Worked for me .... Thank you ...