Update basic_usage.md
Update basic_usage.md

Rewrote the complete grid restriction of B-splines.

Added note on the restriction/importance of 
#samples >= #variables for polynomial regression.
bgrimstad/splinter,bgrimstad/splinter,bgrimstad/splinter,ds283/splinter,bgrimstad/splinter,ds283/splinter,ds283/splinter,ds283/splinter,bgrimstad/splinter,ds283/splinter