Fix a bug related to the python bindings of the analysis backend

Also add a test to catch these problems early

.gitignore

@@ -42,6 +42,8 @@ test/**/last_conf.dat
 test/**/*log.dat
 test/**/trajectory.dat
 test/**/time.dat
+test/OXPY/first_nucleotide.dat
+test/OXPY/last_nucleotide.dat
 
 **/*pyc
 

src/Backends/AnalysisBackend.h

@@ -52,8 +52,8 @@ class AnalysisBackend: public SimBackend {
 
 	const FlattenedConfigInfo &flattened_conf();
 
-	ConfigInfo *config_info() {
-		return _config_info;
+	std::shared_ptr<ConfigInfo> config_info() {
+		return CONFIG_INFO;
 	}
 
 	llint get_conf_step() {

test/OXPY/first_nucleotide_correct.dat

@@ -0,0 +1,10 @@
+[-11.78911323  43.40048831   1.57645506]
+[-14.14325234  40.56472505   2.0471197 ]
+[-18.32734558  37.03027119   2.42922245]
+[-23.77341355  35.3688551    1.82196622]
+[-30.6031531   34.80713034   4.54497927]
+[-34.02963687  36.94788357   9.20757146]
+[-38.71207711  36.98956237   6.66388904]
+[-37.41612362  38.95930485   5.18552101]
+[-35.66393047  38.5800238    2.98663143]
+[-38.16168017  34.77034427   5.37125748]

test/OXPY/input_analysis

@@ -0,0 +1,24 @@
+backend = CPU
+
+steps = 1e6
+newtonian_steps = 103
+diff_coeff = 2.50
+#pt = 0.1
+thermostat = john
+
+T = 20C 
+dt = 0.005
+verlet_skin = 0.05
+
+topology = dsdna8.top
+conf_file = non_existing_conf.dat
+trajectory_file = trajectory_analysis.dat
+refresh_vel = true
+log_file = log.dat
+no_stdout_energy = true
+restart_step_counter = true
+energy_file = energy.dat
+print_conf_interval = 1e5
+print_energy_every = 1e3 
+time_scale = linear
+external_forces = false

test/OXPY/oxpy_compare

@@ -1 +1,2 @@
 DiffFiles::last_nucleotide_correct.dat::last_nucleotide.dat
+DiffFiles::first_nucleotide_correct.dat::first_nucleotide.dat

test/OXPY/oxpy_input.py

@@ -26,4 +26,14 @@ def get_output_string(self, curr_step):
     # run 1k steps
     manager.run(1000)
 
-
+with oxpy.Context():
+    inp = oxpy.InputFile()
+    inp.init_from_filename("input_analysis")
+    # this object will make it possible to access the trajectory data
+    backend = oxpy.analysis.AnalysisBackend(inp)
+
+    with open("first_nucleotide.dat", "w") as f:
+        # loop over all the configurations stored in the trajectory file
+        while backend.read_next_configuration():
+            # print the position of the first nucleotide
+            print(backend.config_info().particles()[0].pos, file=f)