Use macrostrat to get interval data

[willgearty]

Can get many more timescales through MacroStrat (list here), like velociraptr does here:
https://github.com/paleobiodb/analysis_tools/blob/master/velociraptr/R/downloadTime.R

[willgearty]

I'm hesitant to make the package dependent on having an internet connection, so maybe keep the ICS datasets built-in, but also let the user specify any of the macrostrat timescales and attempt to download them and reformat them for use.