Modernisation tasklist

[johnhaddon]

Mega-ticket to track a list of modernisations we want to make to Cortex. I'll keep adding to this as things occur to me. We can split off into separate tickets as and when that becomes useful/necessary.

• Remove Python 2 compatibility
• Use fmtlib
  • Remove all use of boost::format()
  • Add custom formatters for InternedString and other relevant types
  • Add built-in format syntax to IECore::msg()
• Remove DespatchTypedData.h in favour of DataAlgo::dispatch()
• Add move constructors where relevant, particularly for TypedData
• Remove all ops, replacing with Algo where relevant
• Remove old Renderer API. Requires IECoreMaya to stop using IECoreGL::Renderer
• Replace boost with std where possible - optional, variant, function etc
• Use #pragma once