Change default pricing strategy for SED+SVD commodities

[alexdewar]

There are problems with the current default approach (shadow), so let's change it.

@ahawkes has suggested changing it to marginal for now: #1105 (comment)

The major downside of this change is that the marginal and full strategies don't work with circularities (#1168) and circularities are apparantly pretty common in real models. In the short-term, this means we will need to manually set the pricing strategy of commodities with circularities in the circularities example to shadow in order that the associated regression test continues to run. Longer term, we probably want most/all of the examples to use the default strategy.