PixelHistogram (#317)

Multi threaded filling of per pixel histograms for example for detector calibration

1. PixelHistogram - Generic variant expects already pedestal subtracted
data
2. PedestalTrackingHistogram - Terrible name, useful class. Keeps it's
own pedestal and does conversion and pedestal tracking in the worker
threads.

---------

Co-authored-by: Lars Erik Fröjd <froejdh_e@pc-jungfrau-02.psi.ch>

docs/src/python/histogram/index.rst

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+Histogram
+=========
+
+.. toctree::
+   :caption: Histogram
+   :maxdepth: 1
+
+   pyPixelHistogram
+   pyPedestalTrackingPixelHistogram

docs/src/python/histogram/pyPedestalTrackingPixelHistogram.rst

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+PedestalTrackingPixelHistogram
+==============================
+
+.. warning::
+
+    ``PedestalTrackingPixelHistogram`` is specifically designed for use in the Jungfrau calibration
+    pipeline. Make sure you understand the behaviour before using it in other contexts.
+
+``PedestalTrackingPixelHistogram`` accumulates a pixel-wise histogram of
+``frame - pedestal`` residuals while maintaining a running per-pixel pedestal
+estimate.
+
+Use ``push_pedestal_no_update()`` to seed the pedestal estimate, then
+``update_mean()`` before submitting frames with ``fill_async()``. Pending
+asynchronous fills are drained by ``flush()``, and snapshot methods such as
+``values()`` and ``pedestal_mean()`` return numpy arrays.
+
+.. py:currentmodule:: aare
+
+.. autoclass:: PedestalTrackingPixelHistogram
+    :members:
+    :undoc-members:
+    :show-inheritance:
+    :inherited-members:

docs/src/python/histogram/pyPixelHistogram.rst

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+PixelHistogram
+==============
+
+``PixelHistogram`` accumulates one histogram per detector pixel from 2D
+``float64`` images. The public ``PixelHistogram`` name is an alias for
+``PixelHistogram_d``, which stores bin counts as ``float64``.
+
+.. note::
+
+    ``PixelHistogram`` is designed for fast filling from 2D images, utilizing multiple threads, 
+    and contiguous storage of a specific type. No over/underflow bins are provided. For generic use cases consider
+    boost-histogram. (https://boost-histogram.readthedocs.io/en/latest/)
+
+
+Storage-specific variants are also available when a smaller or integer count
+type is preferred:
+
+* ``PixelHistogram_d``: ``float64`` storage
+* ``PixelHistogram_f``: ``float32`` storage
+* ``PixelHistogram_u64``: ``uint64`` storage
+* ``PixelHistogram_u32``: ``uint32`` storage
+* ``PixelHistogram_u16``: ``uint16`` storage
+* ``PixelHistogram_u8``: ``uint8`` storage
+
+.. py:currentmodule:: aare
+
+.. autoclass:: PixelHistogram
+    :members:
+    :undoc-members:
+    :show-inheritance:
+    :inherited-members:
+
+Showing API for PixelHistogram_d, all variant share the same API.
+
+.. autoclass:: aare._aare.PixelHistogram_d
+    :members:
+    :undoc-members:
+    :show-inheritance:
+    :inherited-members:
+    :noindex:
+