Merge pull request #469 from sbmlteam/only-call-loadPlugins-once

Merge pull request #477 from sbmlteam/remove-warnings

Merge pull request #478 from sbmlteam/development

Undo newline change for merge.

Consistent newlines.

Merge pull request #475 from sbmlteam/modelhistory-parent

Replace connectToParent with making the 'parent' functions public.

- make check for gzopen optional

Allow building zlib and libsbml at the same time.

Revert: we explicitly test this.

Revert inadvertent commit.

Remove warnings

Allow a model history parent to be set.

- bump to 5.21.1

Fix comments.