Add analysis tuts

Author: hyanwong

analysing_tree_sequences.md

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+---
+jupytext:
+  text_representation:
+    extension: .md
+    format_name: myst
+    format_version: 0.12
+    jupytext_version: 1.9.1
+kernelspec:
+  display_name: Python 3
+  language: python
+  name: python3
+---
+
+```{currentmodule} tskit
+```
+
+(sec_analysing_tree_sequences)=
+
+# Analysing tree sequences
+
+:::{todo}
+Add all stats material from https://tskit.dev/tskit/docs/stable/tutorial.html
+
+Should we rename this "calculating statistics", or something else with the word "statistics" in the name?
+:::