Myrs should be Myr

[Iddingsite]

I believe Myrs as a unit should be replaced by Myr, which is the commonly used abbreviation (see for instance https://en.wikipedia.org/wiki/Million_years_ago), which caused me some confusion the first time. All the other units are using conventional abbreviations and I think it would be nice to be consistent there.

That's something very easy to fix in the code but it would be a breaking change as it is part of the exposed API, so I don't know if that is a change you want to do now, but for me it should be ultimately corrected.

[boriskaus]

I'm fine with deprecating Myrs. Perhaps introduce Myr and put a depreciation warning for some time that this will be removed in the future. I believe there is a Julia command for that.

[luraess]

Yes it has

[Iddingsite]

Solved by #246.