age gridding -- put all sample points in a single file

[michaelchin]

Just a minor point: in future if you add more than just seafloor ages to the sample points (eg, spreading rate/obliquity/etc) then it might be easier to view if all sample points are in a single file – only because with hundreds of file/layers (ie, one per time step) you’d have to go into each layer and select spreading rate, for example, if you wanted to switch from viewing seafloor age to spreading rate. And if that single file is too big then could always have one file every 100Myr or so, for example.

Originally posted by @jcannon-gplates in #182 (comment)