Masked out tracer age map

Masked out low variance data points from map defined where tracer value
< 1e-4.
Updated figure caption to reflect changes to colorscale and masking.
Iteration 1 edit dcn

First iteration edits based on recommendations from David Noone.
Updated figure labeling
Sect 3.3/4 it 1 edit mcb

Iteration 1 edit for sections 3.3 and 3.4 according to recommendations
by Mary Barth