FEAT: implement `masses` and `invariants` attributes

[redeboer]

Closes #336

• The AmplitudeModel object returned by e.g. published_model() now contains two new attributes, masses and invariants, which makes it easier to create a data transformer for input data that only has two Mandelstam variables, e.g. $\sigma1 = m^2{\pi K}$ and $\sigma2 = m^2{K p}$.
• An example of how to use these new attributes is shown in this section.

[redeboer]

@mmikhasenko I added the snippets from our discussion in #336 on the main page. A few things I don't like:

• The data input seems a rather specific use case (that does not have much to do with polarimetry) and should not be on the main page. On the other hand, one could consider it a short illustration of how to use the PyPI package.
• As discussed, functionality has to be provided to make the workflow more concise. But then again, it seems to me to be a rather specific use-case.
• [Practical point]: The test data points in the ROOT file are all the same

[mmikhasenko]

Thanks! Agree with everything you say.

• The notebook with serialisation seems better place for it.
• I should update the root file: same items look indeed strange
• I would not bother making it more concise right now, would just replace the line '''python m1, m2, .... = symbols(nonnegative) ''' With a call from the module, to avoid user guessing on internal assumptions and names

[redeboer]

• The notebook with serialisation seems better place for it.

Okay good idea, moving the snippet to a new section there.

• I should update the root file: same items look indeed strange

Well it's just an example, I'll generate one with generate_phasespace_sample() and post it here so it can be downloaded in the example

from zipfile import ZipFile
import uproot
import polarimetry
from polarimetry.data import generate_phasespace_sample

model = polarimetry.published_model()
phsp = generate_phasespace_sample(model.decay, n_events=8, seed=0)
with uproot.recreate("lc2pkpi_phsp.root") as root_file:
    root_file["Lc2ppiK"] = {
        "msq_piK": phsp["sigma1"],
        "msq_Kp": phsp["sigma2"],
    }
with ZipFile("lc2pkpi_phsp.zip", "w") as zip_file:
    zip_file.write("lc2pkpi_phsp.root")

lc2pkpi_phsp.zip

• With a call from the module, to avoid user guessing on internal assumptions and names

Yeah would have to provide a small function to do that, but should be doable.