Add macro-particle weight to Coords struct

[ndwang]

weight is a weighting factor typically used when macro-particles are being simulated. It is optional and defaults to nothing.

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Codecov Report

❌ Patch coverage is 60.00000% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/types.jl	60.00%	4 Missing ⚠️

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[mattsignorelli]

In what cases do particles of a bunch (one species) each have different weights?

[ndwang]

In what cases do particles of a bunch (one species) each have different weights?

1. Instead of using macro-particle density to represent a distribution, you can have macro-particles on a fixed grid and use weights. This avoids counting noise.
2. When you have a weak halo, you don't want to track 1 billion particles in the core just so you can have a reasonable number of particles on the edge.

[DavidSagan]

Weights can also be used, for example, when simulating space charge, where the weight is used to calculate how large a wake is left by a macroparticle. A typical situation is to have a leading particle with a large weight which generates a reasonable wake followed by "test" particles with zero weights to not perturbe the wake.

[mattsignorelli]

sounds good.

we should probably just use Base.@kwdef at this point for coords (requiring only v), and then let the Bunch ctor also be all keywords in the same way. This will make initializing a bunch less awkward, e.g. Bunch(v, nothing, weights) instead would be Bunch(v=v, weights=weights). However if we also remove the old ctors, we would have to make sure to bump breaking change version