Variorum 0.4.0

• Add support for ARM Juno architecture
• Add top-level API to query available frequencies, add support for Intel and ARM
• Add JANSSON-C dependency
• Introduce JSON API to return monitoring data into a structure
• Update variorum to build and run with both msr-safe < v1.5 (msr_whitelist)
  msr-safe >= v1.5 (msr_allowlist)
• Add best effort node power cap support for Intel Broadwell and IBM
• Build option to enable all warnings, cleanup compiler warnings
• Default build type switched from Release to RelWithDebInfo
• Consistent formatting for verbose printouts across architectures
• Update year in license headers
• Update docs throughout: JSON API, best effort power cap, architectures
  supported, add sample output to examples
• Rename set functions to cap throughout top-level API (and internally in
  architecture-level functions) to clarify that the setting is an upper bound;
  instantaneous reads on these metrics may be at or below the value