Pdfstream

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**Added:**

* Add `CalibrationExporter`, a callback to export the calibration metadata in poni files.

* Add `xpdvis` server and `xpdsave` server so that the data processing, visualization and exporting can be run in parallel.

* Add `chi_2theta` in the outputs.

* Add yaml metadata output.

**Changed:**

* Data emitted from AnalysisStream changes from numpy objects like numpy.ndarray, numpy.float64 to built-in python objects like list, float.

* Reformat the exported directory structure so that it will be like the outputs from `xpdAn`.

* The format of the output files will be the same as those generated by `xpdAn`.

**Fixed:**

* Fixed the bugs encountered at the 28-ID-1 beamline.

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**Added:**

* Add AreaDetectorTiffHandler for the xpd and lsq servers.

**Changed:**

* The image data can be any array with dimensions N as long as N >= 2. The first N - 2 dimensions will be averaged.

* Simplify the configuration for the servers.

**Deprecated:**

* Deprecate the background subtraction functionality because of the stability.

**Fixed:**

* Use v1 databroker interface for the query of dark images info due to the broken xarray conversion in v2 databroker.

* Fix the bug that server cannot deal with the data for which the background measurement failed.

====================

**Changed:**

* If there is no "calibration_md" in start document, the server will still process the diffraction image but doesn't do the integration and following step. It will use zero for the results depending on the calibration.

* Move the data processing step to the ``process_data`` method in ``AnalysisStream``.

====================

**Added:**

* Allow users to use their own mask by adding the file path in the metadata of the run

* Allow users to disable auto masking by using the metadata of the run

====================

**Added:**

* An xpdvis server that plots figures of analyzed data from xpd server.

====================

**Added:**

* Add the functionality to export files in xpdan style file structure for the xpd server

* More messages from the server including what is running and the errors from pyFAI calibration

**Changed:**

* Average cli check if the directory exits, make it if not.

* AnalysisStream injects the pdfstream version into the start document.

**Fixed:**

* Fix the bug that the plot setting doesn't work in cli.

* Fix the bug about calibration in xpd server.