Clean grid code, probe Tycho data resolution errs

Investigate resolution/"box length"/rminarc errors coming up in Tycho
data. Plans to work around errors.
Add parameters to model code, docstrings etc; update SNR catalog data.
Split code for executing fits/table calculations.
Start adding material for Tycho's azimuth dependent shock speeds.
Add misc notes on v_shock/d_remnant dependence.