Release Candidate for the 2.1.0 release of the VIRL Client Library.

Changes since 2.0.0

• added additional examples (licensing, link conditioning)
• default username and password have been changed from virl2 to cml2 and cml2cml2, respectively
• entire code base is using Black for formatting
• link conditioning API added to library
• node CPU limiting added to library
• licensing added to library
• controller version checking done when connecting