Handle numerically unstable calculations

[jacanchaplais]

Approaches:

1. Raise warnings
2. Use sentinels, eg. NaN
3. Clamp values to sensible defaults, eg. 1.0e-14 -> 0

Introducing NaNs seems to be a good idea for MomentumArray.phi when transverse momentum is very small. Then again, emitting particles with negative masses if the magnitude of spatial momentum > energy seems to be common. I suspect these particles generally just have quite a close cancellation between momentum and energy, so it's a numerical precision issue.

Raising warnings would avoid potential breaking changes, but would clutter the standard output, and give users little ability to automatically handle the invalid data.

Clamping values seems risky, because this would assume that the user is happy to have the data get cleaned up for them.

Or nothing could be done, and the impetus could be put on the user to do their own data checking and validation. This seems a bit disingenuous, though.

Will place issue on the backlog until an approach is decided upon.

[jacanchaplais]

Will implement both warnings and NaNs.