v2.1.0

- Binaries are now produced with the [AppVeyor service](https://ci.appveyor.com/project/quesnel/vle?branch=master).
- Switched from travis service to [Github Actions](https://github.com/vle-forge/vle/actions) the Linux continuous integration.
- Switched from libxml2 to libexpat1.
- Added the possibility to use secure or unsecure package download.
- Added a `--workspace-prefix-path|-w` parameter to CVLE to simplify bash scripting.
- Added a new API for `vle::manager` usable for Python, R and command line interface.
- Added a new class `vle::devs::MultiComponent`  to develop MultiComponent models.
- Improved API and ABI.
- Fixed varions warnings and compilation issues