Coherent policy needed for use of `jax.jit`

[bwohlberg]

A coherent policy for the use of jax.jit needs to be formulated and implemented. At present, its usage seems somewhat arbitrary and inconsistent; for example:

1. Operator defaults to jit=False while LinearOperator defaults to jit=True
2. For the most part there is no jitting of Functional classes, with the exception of L0Norm.prox.
3. Refactoring of methods in scico.optimize classes to support jitting should be explored.

[bwohlberg]

A suitable place should be found in the docs for the jax notes on the use of jit.

[bwohlberg]

Suggestion for a coherent policy made during a discussion of #467: all functions that can be jitted should be jitted at the point of definition using the @jax.jit decorator.