ND2: only populate the channel color if it is non-zero

Setting the channel color to 0 (i.e. black) can prevent the images from
displaying correctly when the color-appropriate lookup tables are applied.
fix SimplePCI links in documentation
Merge pull request #719 from mtbc/fix-simplepci-link

fix SimplePCI links in documentation
Merge pull request #717 from melissalinkert/nd2-channel-color

ND2: only populate the channel color if it is non-zero