Fix mixed numeric sum aggregation

[mattfaltyn]

Summary

• combine integer and float subtotals as a double in #sum
• add a focused regression test for mixed integer/float input

Testing

• cargo test
• cargo fmt --all -- --check
• cargo clippy -p nemo-physical -p nemo --all-targets -- --deny warnings --allow clippy::nonstandard_macro_braces --allow clippy::double_must_use

The two allowed clippy lints are existing diagnostics in unrelated files; the changed target is warning-free.

Fixes #800.

[aannleax]

Looks good, thank you :)

[aannleax]

I will fix the unrelated clippy error in another PR