replaced instances of 'reference' with NP instances
replaced instances of 'reference' with NP instances

JacquesCarette/literate-scientific-software,JacquesCarette/literate-scientific-software,JacquesCarette/literate-scientific-software,JacquesCarette/literate-scientific-software,JacquesCarette/literate-scientific-software,JacquesCarette/literate-scientific-software,JacquesCarette/literate-scientific-software