Remove Time from ParallelInterpolation

[knelli2]

Proposed changes

Use some template magic.

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Further comments

[nilsvu]

LGTM. I suppose another option would be to require all "temporal IDs" can convert to double (with explicit operator double()). @wthrowe what do you think?

[wthrowe]

Interesting idea. I don't see anything obviously wrong with it. There's only one sane interpretation of a TimeStepId as a double, and there aren't many places where both types would be valid even if you did get confused about converting.

Go ahead if you want to try it. Shouldn't be too hard to back out if we find it causes a problem.