McLachlan: Make finite difference order and use of Jacobians selectable at runtime

Currently, we generate separate !McLachlan thorns for different finite differencing orders and for single-patch and multi-patch. The attached patch makes use of a new feature in Kranc to enable all finite differencing orders in all thorns, as well as enabling multi-patch support in all thorns. These features can now be selected in all thorns via run-time parameters (fdOrder and jacobian_group). Each thorn defaults to the finite differencing order that it used before.

This change should be entirely backwards-compatible so users don't need to change their parameter files or thornlists immediately.

Use of any thorn other than ML_BSSN could be deprecated immediately, and people should start to transition to using only ML_BSSN. At some point in the future (after the next release), the other thorns will be dropped. I don't want to drop the other thorns immediately, as parameter files would have to be changed, and that makes comparing parameter files between runs very tedious.

We need to decide whether we want to keep the _UPW variants. These cannot be used with vectorisation.

Keyword: McLachlan

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