Break on EOF when an input mode is specified

Typically a filter program will exit when stdin closes, which allows pipelines
to close gracefully when the sources dry up.  When an input mode is specified
(ie -AFGgW or -k, which is effectively an input mode) then timeseries acts as a
filter and so should close in this manner.
Add methods to solve for start_time given n_steps and period
Add solver for period given start, stop, n_steps
Merge branch 'expanded_solvers'
Merge branch 'master' into cleanup
Add tests for solving for period, start time