add two datasets converted from NP-hard problems

Author: yaohuicai

data/maxsat/README.md

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+E-Graphs converted from MaxSAT problem according to method mentioned in the [paper](https://escholarship.org/uc/item/85f640cc).
+The MaxSAT dataset is [MaxSAT Evaluations](https://journals.sagepub.com/doi/abs/10.3233/SAT190047).
+
+This dataset was originally used in [SmoothE: A differentiable E-Graph Extraction](https://dl.acm.org/doi/10.1145/3669940.3707262).

data/maxsat/hamming6-2.clq.json

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+E-Graphs converted from minimum set cover problem according to method mentioned in the [blog](https://effect.systems/blog/egraph-extraction.html).
+The set cover dataset is [Frequent Itemset Mining
+Dataset Repository](http://fimi.uantwerpen.be/data/).
+
+This dataset was originally used in [SmoothE: A differentiable E-Graph Extraction](https://dl.acm.org/doi/10.1145/3669940.3707262).