add "longerwider" function w/tolerance

[dchud]

David and team at CDC have used a variation on longwide() which allows for a rolling time window specified in days as a tolerance for aligning heights and weights into "matched" observations for the purposes of calculating BMI, etc.

#71 addresses enhancements to the existing longwide() which won't fundamentally change how it behaves. Consider adding a new function designed to work with a time window.