Add ML training pipeline with Polaris optimizer

- Add polaris dep for Adam optimizer (Axon.Optimizers deprecated)
- Fix Model.train/3 to use Polaris.Optimizers.adam
- Training task: mix propagation_train with stratified band sampling
- Model encodes solar time (longitude/15) not fixed timezone

mix.exs

@@ -67,7 +67,8 @@ defmodule Microwaveprop.MixProject do
       {:earmark, "~> 1.4"},
       {:nx, "~> 0.9"},
       {:axon, "~> 0.7"},
-      {:exla, "~> 0.9"}
+      {:exla, "~> 0.9"},
+      {:polaris, "~> 0.1"}
     ]
   end