flake: Add PyThermostat to devShell

Make Python scripts in PyThermostat available in the development shell
(thermostat_plot, thermostat_autotune, and thermostat_test).

flake.nix

@@ -94,7 +94,11 @@
           ++ (with python3Packages; [
             numpy
             matplotlib
+            pythermostat
           ]);
+        shellHook = ''
+          export PYTHONPATH=`git rev-parse --show-toplevel`/pythermostat:$PYTHONPATH
+        '';
       };
 
       formatter.x86_64-linux = nixpkgs.legacyPackages.x86_64-linux.nixfmt-rfc-style;