pyln: Rename LightningNode.fund_channel to fundchannel

It is inconsistent with the RPC method, which was bothering me for some time.

contrib/pyln-testing/pyln/testing/utils.py

@@ -739,7 +739,11 @@ class LightningNode(object):
         self.start()
 
     def fund_channel(self, l2, amount, wait_for_active=True, announce_channel=True):
+        warnings.warn("LightningNode.fund_channel is deprecated in favor of "
+                      "LightningNode.fundchannel", category=DeprecationWarning)
+        return self.fundchannel(l2, amount, wait_for_active, announce_channel)
 
+    def fundchannel(self, l2, amount, wait_for_active=True, announce_channel=True):
         # Give yourself some funds to work with
         addr = self.rpc.newaddr()['bech32']
         self.bitcoin.rpc.sendtoaddress(addr, (amount + 1000000) / 10**8)