- stretch/vmid/exponent for each channel in rgb
- stretch/vmid/exponent for each channel in rgb

git-svn-id: 83a0274dba4329d458142e4079227af35a786d9d@102 2f3f69f2-b874-4715-96d5-35985bcb53a6

allisony/aplpy,mwcraig/aplpy