Refactor plot helper function

Cleans up the parsing of the `setplot` argument and adds more uniform
behavior with the `plot_kargs` argument.
Merge pull request #361 from mandli/refactor-plot-func

Refactor plot helper function
Fix doc string of solution read method
Fix binary output bug related to read_data_line.

This bug was introduced when `read_data_line` was refactored in PR
"Clean up I/O" \#358 where the function interface changed from `type`
to `data_type` and no longer used strings to describe the return type.
Merge pull request #362 from mandli/binary-read-data-bug-fix

Fix binary output bug related to read_data_line.
Change _rk_stages to _registers
Add SSPMS(3,2) and general linear multistep time-discretization.
Edit linear multistep methods.
Adding variable time-step functionality to SSPMS32, editing LMM, adding a dictionary for ssp coefficients in sharpclaw class.
Corrections so that classic solver runs.
Introduce generic functions for cfl_max and dt_new.
Merging from ketch/linear_multistep_methods.