Multiple model outputs

[remicres]

Currently, model outputs of same size are handled in inference.

Outputs parameters

A nice enhancement could be to add a parameter group for models outputs, and use the same strategy as the input sources.
For instance, an environment variable called OTB_TF_NOUTPUTS could set the number of model outputs.

Each output would have:

• a name (output1.names would replace outputs.names)
• an expression field (output1.efieldx/y would replace outputs.efieldx/y)
• a scale factor (output1.spcscale would replace outputs.spcscale)

Outputs writing

We should use the new otb::MultiImageFileWriter.

See here how they implemented that in the Orfeo ToolBox!

API Change

Breaks main API so milestone is OTBTF 3.0