CKDSpectralContext: Add default bin set, remove direct constructor calls

[leroyvn]

Description

This PR sets more complete defaults for the CKDSpectralContext class. The missing default value for the bin_set parameter would indeed be problematic when passing an instance missing a value.

A few tests are refactored for improved scoping. Direct calls to the CKDSpectralContext() constructor are also removed (a SpectralContext.new() is always preferred).

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 give permission that the Eradiate project may redistribute my contributions under the terms of its license