Merge pull request #13 from SpinW/11-add-design-decisions-draft-for-e…

…xperiment-setup

Draft design decision for macroscopic structure

docs/developers/adr/002_structure_for_describing_macroscopic_structure.md

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+# A Structure for Describing Macroscopic Structure
+
+## Context
+
+The main job of spinW is to calculate the energy levels of a magnetic system over a range of q values. This
+task is relatively self-contained and well defined by just the material's cystallographic and magnetic properties.
+
+However, there are a number of things that are more properly understood as a property of a particular experiment. 
+The main features are to include here is whether or not the crystal is twinned or is a powder.
+
+There is a question about whether it should be the location for the option to apply neutron scattering selection rules.
+
+
+## Decision
+
+We will create a base `Sample` class with derived `TwinnedSample` and `PowderSample` classes which will have methods for preprocessing the input **Q**-vector list and post-processing the output spectra from `spinwave()` for the case of twinned crystals and powder (polycrystalline) samples. The base class will be used by default for untwinned crystals.
+
+## Status
+
+Accepted
+
+## Consequences
+
+This will add another new object that users are unfamiliar with. (see backwards compatability proposal)