Recursive interpolation

[BrendanKKrueger]

PR Summary

Recursive implementation of interpToReal to avoid the big, nasty formulas.

PR Checklist

• [x] Code is formatted. (You can use the format_spiner make target.)
• [ ] Adds a test for any bugs fixed. Adds tests for new features.
• [ ] If preparing for a new release, update the version in cmake.

[BrendanKKrueger]

@Yurlungur, I give about a 50% chance that this solution is controversial. But it's a way to clean up all the nasty equations that span 30ish lines. It also extends the ability to use an index in place of a coordinate so that it can be applied to any dimension instead of only the hard-coded ones (assuming one uses the new interface rather than the existing interface that I left in place so that we discuss all the details that go into this change).

[BrendanKKrueger]

It also extends the ability to use an index in place of a coordinate so that it can be applied to any dimension instead of only the hard-coded ones

It may be that this fulfills your desire for DataBox to be used as a data array in addition to being used as an interpolator. I realize I'm proposing more sweeping changes, but this is just thinking about possible future evolution. If we rename interpToReal to operator() then the user can do something like

auto value = db(1,7,3);

and that becomes simply looking up the value with those indices. If the user instead does

auto value = db(1.0, 7.0, 3.0);

then that's an interpolation. And they can mix and match as well, such as

auto value = db(1.0, 7, 3.0);

Of course, one could argue that this becomes unclear and hard to understand the difference between interpolating and indexing, but maybe not.