[feat] example for loading local blockchain

examples/load_local_blockchain.py

+# this example lets you load locally copy of duniter blockchain into duniterpy objects
+# by default, it searches in ~/.config/duniter/duniter_default/g1/
+
+from duniterpy.helpers.blockchain import load
+
+bc = load() # gets blockchain iterator
+b = next(bc) # gets block
+b.number # should return 0
+# you can access all properties of this block through it's duniterpy objects attributes
+next(bc).number # should return 1
+next(bc).number # should return 2 (and so on)
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