Change the API to more idiomatic Scala

[PawelLipski]

Non-idiomatic stuff to change:

Should:

• [x] def astype[U](newType: NumPyType[U]): NDArray[U] + def float32: NumPyType[Float]
• [x] method/val naming (as for now they have underscores etc.)
• [x] model, accuracy, sigmoid - make enums instead of strings
• [x] union types => implicit Reader instance for Unions - auto derivation (?)
• [x] distinction b/w traits for Python modules and for Python classes
• [x] _enter_() - with contexts
• [x] package structure (tensorflow.*)
• [x] def AdamOptimizer(lr: Double): Optimizer => def AdamOptimizer(lr: Double): AdamOptimizer etc.
• [x] s/Example/GradientDescentOptimizerExample for consistency
• [x] wire up scalafmt

Could:

• [ ] apply_gradients (and maybe other operations) are mutable - they change Variables
• [ ] NDArray - make more type safe? not clear how this can be done

[PawelLipski]

~the examples are almost identical for Scala 2.x and 3.x, possibly their code could be shared on some level~ moved to a separate issue