Version 2.1.0

From News.md:

• All examples are within the \donttest tag (see #100).
• get_collection() methods always return an object of class mgNetworksCollection (see #100).
• get_network_by_id() gains an argument force_collection to force the class collection (see #100).
• Vignette now precomputed (see #100).
• Tests now use vcr (see #100).
• Travis and Appveyor removed, use GitHub Actions (see #100).
• avail_type() is no longer exported.