[skip-ci] Update the User's Guide to address #80

[rogerfraser]

Overview

This pull request addresses #80 by updating the User's Guide with the following:

• Changes throughout to reflect the default reference frame of GDA2020. Note that certain references to GDA94 in the examples remain in accordance with the use of GDA94 in the sample files.
• Introduce references to AUSGeoid2020 and the Australian Gravimetric Quasigeoid (AGQG)
• Update bibliography with the following:
  • Add the Australian Vertical Working Surface (AVWS) Technical Implementation Plan (1.6)
  • Update the Geocentric Datum of Australia 2020 Technical Manual (1.6)

Many thanks @jhaasdyk-au for your feedback and suggestions.

[coveralls]

Coverage increased (+0.02%) to 79.426% when pulling 72c3617948a41db7f4f1b21288c1fc36abe202dc on rogerfraser:master into 9cbcf2c641f2b63f7de7758a5f128e3a11ad9712 on icsm-au:master.