Tensor sparsity metric

[kylesayrs]

Implements sparsity metric for initializer tensors as a first step to supporting #1240 and #65.

• Add style member to view.Argument to store style
• Implement the percentage style for displaying percentages such as sparsity
• Add a metrics section to the TensorSidebar which displays tensor metrics
• Lazily calculate metrics of a tensor only when demanded by TensorSidebar

Tensor Properties sidebar now includes a metrics section with sparsity

Tensors which are not initializers do not have metrics