Make libSBML well consumable via FetchContent

[starboerg]

Adding a missing (sbml) library via cmake's FetchContent is a convenient choice, in particular since cmake v3.24 .

In that case, some targets should be hidden (e.g. uninstall, tests, examples and so on ).
Unnecessary targets should be dropped if PROJECT_SOURCE_DIR and CMAKE_SOURCE_DIR are not identical.

For more details see e.g. FetchContent Tutorial.
This could safe us from patch-renaming conflicting targets and creating the SBML::SBML target ALIAS.

Thanks, the Morpheus team.