Produce release v.2.0.1

[pedrogit]

Tested with PostgreSQL v13.1 and v11.3

Only minor issues:

• Better support for NULLs in TT_PadConcat() (PadConcat Null bug #248)
• Added an order parameter to TT_GetIndexTestVal() (PadConcat Null bug #248)
• Fixed an issue with TT_SubstringText() making some translation very slow (Some LEFT JOIN generated by the lookupText() helper function are VERY slow #250)
• Fixed a minor issue in TT_PrettyDuration() for durations like 111.2973 and 59.9
• Display MEAN TIME PER ROW in TT_ShowProgress() only when currentRowNb > 1
• Added default error codes for some helper functions
• Minor changes to the main Readme
• Added citing instructions
• Added a license file (GNU)