Handle pixels that never reach threshold GDD

[Aariq]

This addresses #31

• Use Inf instead of NA to represent a pixel that never reaches a threshold GDD
• In plots of DOY and normals mean DOY, lump those Infs in with the max DOY and plot as "≥{max DOY}" in the colorbar
• Even when there aren't Infs, always plot the max DOY (for consistency).
• When calculating pixel-wise linear regressions, only use pixels if at least half the years have non-Inf values and drop any Infs before fitting.