Fix IndexError when no lags given in command

Previously, if no lags of the dependent variable were included, you
would get an `IndexError` on line 313, as it would be the case that
`dep_lags = [0]`, which makes `len(dep_lags) == 1`, so it would be
impossible to index `dep_lags[1]`.

That is, if no lags are included, `len(dep_lags) == 1`. Line 309 has
been updated to reflect this fact.

Additionally, line 305 could theoretically give an IndexError now,
as there is no check that `len(dep_lags) > 0`. This check was added as
well.

With those two checks, the check for `len(dep_lags) > 0` on line 313
is no longer necessary, as the first two checks guarantee `len(dep_lags)
> 1`.

pydynpd/command.py

@@ -69,7 +69,7 @@ def parse_command(self):
         if len(parts) <= 1:
             print('There should be at least two parts in command string')
             exit()
-        
+
         if len(parts) > 3:
             print('too many parts')
             exit()
@@ -302,15 +302,15 @@ def check_dep_indep(self):
         dep = self._temp_part1_list.names[0]
         dep_lags = self._temp_part1_list.lags[0]
 
-        if dep_lags[0] != 0:
+        if len(dep_lags) > 0 and dep_lags[0] != 0:
             print('dependent variable should not be lagged on the left hand side of the model')
             exit()
 
-        if len(dep_lags) == 0:
+        if len(dep_lags) == 1:
             print('lagged dependent variable should be included')
             exit()
 
-        if len(dep_lags) > 0 and dep_lags[1] != 1:
+        if dep_lags[1] != 1:
             print('lag 1 of the dependent variable is not included')
             exit()
 
@@ -350,11 +350,11 @@ def check_three_lists(self):
                 self._temp_iv_list.insert(var_name, var_lags)
 
     # def check_adjustable(self):
-    # 
+    #
     #     if len(self.adjustable['indep'])>0:
     #         dep = self._temp_part1_list.names[0]
     #         self._temp_part1_list.lags[0]=[1]
-    # 
+    #
     #     for var in self.adjustable['indep']:
     #         if var.name != dep:
     #             print('in the current version, only the lags of the lagged dependent variable can be adjusted')