Initial stub file for architecture.md, as per request of CHES artifac…

…t reviewer (to be detailed further)

ARCHITECTURE.md

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+# Architecture
+
+This document is a minimalist description of SCARR's architecture and its guiding design principles. This is a good place to start to familiarize yourself with the code base.
+
+## Design Principles
+
+* Make sure that code coupling is reduced to the bare minimum needed
+    * Files should be mostly self-contained, if necessary, at the expense of code-redundancy
+    * Avoid syntactic sugar and unneeded layers of abstraction
+    * Goal is an R&D-friendly framework that is easy to extend by others on a per-file basis
+    * This approach also aligns best with the per-file licensing of MPL 2.0
+* Aim for I/O parallelism (high-level vs. low-level) and asynchronous pre-fetch
+    * High-level parallelism: to access the same .zarr in a non-blocking way from multiple threads
+    * Low-level parallelism: to fetch the chunks of a batch in parallel (not sequentially)
+        * ZarrV2: currently does *not* support this, chunks are fetched sequentially with f.read()
+        * ZarrV3: *might* improve this, but otherwise OS-specific approach needed (e.g., io_uring)
+    * Asynchronous prefetch: read the next batch while computation over previous batch takes place
+        * If compute time > I/O time then: async prefetch works
+        * If compute time < I/O time then: async prefetch fails
+    * Simplify scaling by using the buffer cache as opposed to explicitly changing accumulator variables
+* Aim for compute parallelism despite limitations of GIL
+    * Multiprocessing in Python but chunksize=1 (simply to have another GIL)
+    * All other threading using Numba or NumPy (for threading efficiency)
+
+## Bird's Eye View
+
+TBD
+
+## Code Map
+
+TBD
+
+## Cross-Cutting Concerns
+
+TBD
+