Flows to functions

[danielward27]

Breaking change:

• All the flows have moved from being classes (with standard PascalCase naming) to functions with the standard snake_case naming.
• Flow arguments other than the first argument (key) are now key-word only.

Other changes:

• Using properties where possible, in order to avoid cluttering PyTrees with duplicated information.

Reason for the breaking change:

• None of the flows implemented any methods or extended the base class AbstractTransformed in any way, so the use of classes is not needed. Instead, flows now are functions that return a Transformed distribution. By being a class, it implicitly suggests the flows must implement some methods or extend the base class, which is confusing.
• The only advantage of having flows as classes was to allow storing of additional attributes in the class. In general this wasn't particularly useful, and most of the attributes provided duplicate information from the underlying bijection.
• Enforcing key word only arguments for flows ensures readable code.