implement format/expression in input model band definition

[fmigneault]

Description

Allow mlm:input to define bands with object representation:

{
  "name": "band-name",
  "format": "expr-fmt",
  "expression": "{}"
}

Using this, the model can define per-band pre-processing steps, such as, computing a virtual NDVI band consisting of an arithmetic combination of other band sources.

Related Issue

• Fixes How to define an MLM input that depends on a preprocessing operation #7

Type of Change

• 📚 Examples, docs, tutorials or dependencies update;
• 🔧 Bug fix (non-breaking change which fixes an issue);
• 🥂 Improvement (non-breaking change which improves an existing feature);
• 🚀 New feature (non-breaking change which adds functionality);
• 💥 Breaking change (fix or feature that would cause existing functionality to change);
• 🔐 Security fix.

Checklist

• I've read the CONTRIBUTING.md guide;
• I've updated the code style using make codestyle;
• I've written tests for all new methods and classes that I created;
• I've written the docstring in Google format for all the methods and classes that I used.

[rbavery]

Looks great! Just some minor language edits, tests and descriptions are thorough.