Add tests for reading and writing the Metadata

Currently, the tests run by the CI don't allow to check if there is a regression on the Metadata reading/writing process. With the coming of OTB version 8.0 freed from OSSIM, and the new Metadata architecture, we have no way to assess there is no regression, except by checking manually (see this exemple).

We should modify the script of image comparison used in the tests to compare the output to the baseline, so it also checks the metadata. That would mean updating a big part of the baseline, which is a lot of work.

To avoid this big update of the baseline, we could identify some common use-cases that need to be tested, and activate the metadata checking only for this cases. That way, we could cover the Metadata reading/writing process with a small change in the baseline.

1. Implement metadata checking mechanism, test it on some basic applications. (5pts)
2. Chose the applications and filters that are relevant for testing.
3. Set up the tests for theses applications.