Add PointCloud spatial distribution

[gonuke]

Description

Allows users to define a list of points in space, each with a different relative intensity, to be sampled discretely. This is a valid SpatialDistribution and can be used anywhere that a SpatialDistribution is valid.

Fixes #3159

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[pshriwise]

Thanks for this snappy PR @gonuke! A useful distribution that we haven't been able to support before!

[pshriwise]

Suggested change

	psoitions: numpy.ndarray (3xN)
	The points in space to be sampled
	positions: numpy.ndarray
	The points in space to be sampled with shape (N, 3)

[pshriwise]

Suggested change

[pshriwise]

I'm thinking the XML format of the data as a flat list of points (X1, Y1, Z1, X2, Y2, Z2, ...) would make the import export code simpler, and it isn't more or less readable in an XML format (for my brain anyway).

[pshriwise]

Suggested change

	subelement.text = ' '.join(str(e) for e in self.positions[idx,:])
	subelement.text = ' '.join(str(e) for e in self.positions[..., idx])

I think?

[paulromano]

@gonuke Thanks for this PR! Out of curiosity, what is the intended application of this feature?

[gonuke]

We have a user who has an existing approximation of a volumetric source by a high density list of isotropic point sources. They want to use that source for comparison to other simulations.

[gonuke]

While it may have been elegant to add an istream operator for Position the version I added did not round-trip with the existing ostream operator and it is more convenient in XML to not use the verbose ostream format.

Thus, I've relied on just reading a list of doubles and packing them into Position in the reader.

[pshriwise]

Some small line suggestions here @gonuke. I went ahead and added test that samples the external source distribution on the C++ side and factored out a function in the XML interface for parsing a set of Positions from a flat array of values on a node. Hope that's okay!

Thanks again for this addition.

[pshriwise]

I think I'd opt to let the check_iterable_type throw an error here instead.

[pshriwise]

I just pushed a couple of small changes, but this looks good to me! @paulromano, you mentioned you'd like to have a look at this as well, so I'll give you a little time to review.