[Python][UHI] Start introducing UHI serialization

[siliataider]

This Pull request:

This PR introduces the first steps toward enabling UHI serialization for ROOT histograms (see scikit-hep/uhi#167).

It implements

• _to_uhi_ for ROOT histograms: provides an Intermediate Representation of the histogram described by the UHI schema.
• _from_uhi_ classmethod: converts from a UHI histogram to a ROOT histogram.

Minimal reproducer:

import json

import numpy as np
import uhi.io.json

import ROOT

# Create a histogram
h = ROOT.TH1D("h", "h", 10, -5, 5)
h[...] = np.arange(10)

# Serialize to JSON
ir = json.loads(json.dumps(h, default=uhi.io.json.default), object_hook=uhi.io.json.object_hook)

# Deserialize via constructor
h_loaded_ctor = ROOT.TH1D(ir)

# Deserialize via classmethod
h_loaded_clsmethod = ROOT.TH1D._from_uhi_(ir)

assert np.array_equal(h.values(), h_loaded_ctor.values()) and np.array_equal(h.values(), h_loaded_clsmethod.values())

[github-actions]

Test Results

    22 files      22 suites   3d 22h 52m 56s ⏱️
 3 792 tests  3 792 ✅ 0 💤 0 ❌
80 337 runs  80 337 ✅ 0 💤 0 ❌

Results for commit ac2177b.

♻️ This comment has been updated with latest results.

[vepadulano]

Very nice, thanks! A couple of minor comments from my side. As a followup, we'll definitely want to add some docs to the UHI section and possibly a tutorial. That can come later of course, this is also a fresher UHI feature which I believe still needs discussing in general outside of ROOT.

[siliataider]

Thanks for the review @vepadulano , and yes docs will come in a later PR