Throw an error for particles-in-particles

[aplavin]

Presumably, it's never intended to have Particles{T} where T contains Particles inside. But such a construction can accidentally happen, eg when making a mistake in Unitful Particles creation.
Currently, it attempts to compute the result, which can hang for a very long time when there are just 2-3 levels of "Particles nesting" with any nontrivial calculation.

If that's possible at least in some reasonable cases, would be nice to add the guardrail and throw some error immediately on construction.