Add Image/Pixels/Channel/LightSourceSettings

[MatteBailey]

It is important that for each channel, we capture what laser was used and at what power (eg. 43% for EGFP). This information is captured in CZI at ImageDocument/Metadata/Information/Image/Dimensions/Channels/Channel/LightSourcesSettings, but there are a couple complications:

1. CZI allows multiple light sources per channel, while OME only allows one.
2. CZI captures intensity at LightSourcesSettings/LightSourceSettings/Intensity, while Image/Pixels/Channel/LightSourceSettings has an Attenuation attribute. I'm told this is related to intensity, but is different.