Gave plot3.phase() the upgrades that plot3.lc() received in commit a4f4.

Tested; works.
Fixed typo in docstring. ("defalt" is not a word)
Implemented `period_decimal_places` to fulfill swolk's request

where we want to only print 2 decimal places in the period, in our plots.
Added `plot_warn` keyword to plot3.phase(), just like in commit cf86.