Add a CC BY license to the included datasets

examples/example_data/LICENSE.txt

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+The datasets contained in the following three files:
+
+- Optical_Data_ss.cxi
+- Optical_ptycho_incoherent.pickle
+- lab_ptycho_data.cxi
+
+Are made available under a CC BY 4.0 License, defined here: https://creativecommons.org/licenses/by/4.0/
+
+The dataset contained in the file:
+
+- AuBalls_700ms_30nmStep_3_6SS_filter.cxi
+
+is sourced from https://cxidb.org/id-65.html, and was made available by the original authors under the CC0 Public Domain Dedication Waiver. This data was deposited into the CXIDB by Stefano Marchesini.