Incorporate the use of EGM in geoid

[harry093]

Allow DynAdjust to switch to using EGM when outside AUSGeoid2020 extent

[mbremner]

I have an EGM2008.gsb file that I created from the 2.5 minute version of EGM2008. Deflections of vertical are computed by taking the derivatives in the north and east directions of the bi-quadratic interpolation of each point and its 8 surrounding points. The first and last rows and columns are removed because the deflections of vertical could not be calculated due to the lack of surrounding points. The file is quite large though, around 500mb. I am not sure if it should be added in a pull request. Is there a way that I can send this file? If so, would you be able to post it as an asset that interested users could download?

[rogerfraser]

Thanks @mbremner for your work on creating EGM2008.gsb. As mentioned in this comment, our preference is to not store geoid files in this repo, but to direct users to 'point of truth' locations from where they can obtain the latest authoritative version(s).

Where would be the best place to house EGM2008.gsb files?

@JackMcCubbineGA