Spectrum: Allow assigning None to the quantity field

[leroyvn]

This PR closes #333.

Description

This commit allows leaving the Spectrum.quantity field unset. This is required to allow for incomplete Spectrum object initialization, which is, in turn, required to allow for the deferred application of a default quantity field by a converter.

The policy is as follows:

• if quantity is set, unitless spectrum values are applied appropriate units and validators check if spectrum values have consistent units;
• if quantity is None, unitless spectrum values are allowed and validators do not check spectrum value units, which can be anything.

Integral computations will consider unitless fields as dimensionless.

How does it work, in practice?

I refactored the Spectrum constructors to apply units, if relevant, prior to initialization. For example with UniformSpectrum:

• if quantity is set and value is assigned a quantity, the validator makes a consistency check;
• if quantity is set and value is assigned a unitless value, value is attached units, then the validator makes a consistency check;
• if quantity is unset nothing happens.

If one instantiates a UniformSpectrum with unset quantity, it is possible to:

• apply arbitrary units to the value field;
• assign a value to the quantity field.

Both should be done with attrs.evolve() to make sure that validation occurs.

The whole point of it

The SpectrumFactory.converter() method is also updated to allow for consistent initialization of Spectrum objects in situations where a missing quantity field can be inferred from the context.

In practice, these fixes enforce symmetry between spectrum dictionary and constructor syntaxes:

import eradiate.scenes as ertsc

# 1
ground_bsdf_dict = ertsc.bsdfs.LambertianBSDF(
    reflectance = {
        "type": "interpolated",
        "wavelengths": [440, 550, 620],
        "values": [0.067269, 0.096786, 0.125058]
    }
)

# 2
ground_bsdf_obj = ertsc.bsdfs.LambertianBSDF(
    reflectance=ertsc.spectra.InterpolatedSpectrum(
        wavelengths=[440, 550, 620], 
        values=[0.067269, 0.096786, 0.125058]
    )
)

In snippet 1, the conversion protocol completes the dictionary with a "quantity" entry, then used to instantiate a well-formed InterpolatedSpectrum. In snippet 2, an incomplete InterpolatedSpectrum instance is created; the converter then evolves it into a new object for which the quantity field is specified; all unitless fields are then converted and validated.

Checklist

• The code follows the relevant coding guidelines
• The code generates no new warnings
• The code is appropriately documented
• The code is tested to prove its function
• The feature branch is rebased on the current state of the main branch
• I updated the change log if relevant
• I give permission that the Eradiate project may redistribute my contributions under the terms of its license

[leroyvn]

@schunkes This turned out to be harder to fix than I thought. The good thing is that it forced the definition of a policy on when to apply units to unitless fields or not.