converted all images to png

[makedoc]

closes #141

user-doc/tutorials/belfast-6.txt

@@ -375,7 +375,7 @@ without further rescaling needed.
 We can plot the time evolution of the CVs along with the height of the Gaussian.
 
 \anchor belfast-6-wtb6-fig
-\image html belfast-6-wtb6.pdf "Time evolution of the CVs and Gaussian height during the first 2.5 ns of a well-tempered metadynamics simulation with biasfactor equal to 6."
+\image html belfast-6-wtb6.png "Time evolution of the CVs and Gaussian height during the first 2.5 ns of a well-tempered metadynamics simulation with biasfactor equal to 6."
 
 The system is initialized in one of the local minimum where it starts accumulating bias.
 As the simulation progresses and the bias added grows, the Gaussian height is progressively reduced.
@@ -391,7 +391,7 @@ this biasfactor is not large enough to allow for the system to escape from the i
 in the time scale of this simulation.
 
 \anchor belfast-6-wtb15-fig
-\image html belfast-6-wtb15.pdf "Time evolution of the CVs and Gaussian height in a 5 ns long well-tempered metadynamics simulation with biasfactor equal to 1.5."
+\image html belfast-6-wtb15.png "Time evolution of the CVs and Gaussian height in a 5 ns long well-tempered metadynamics simulation with biasfactor equal to 1.5."
 
 Following the procedure described for standard metadynamics in the previous example, 
 we can estimate the free energy as a function of time and monitor the convergence of the
@@ -435,7 +435,7 @@ Let's look at the HILLS file, in particular at the time serie of the CV psi and
 Gaussian height.
 
 \anchor belfast-6-phialone-fig
-\image html belfast-6-phialone.pdf "Time evolution of psi and Gaussian height in a well-tempered metadynamics simulation using psi alone as CV."
+\image html belfast-6-phialone.png "Time evolution of psi and Gaussian height in a well-tempered metadynamics simulation using psi alone as CV."
 
 From this plot, we observe a nice diffusive behavior of the CV psi when the Gaussian height is already
 quite small. This happens until t=3 ns, when the CV seems to be stuck for a while in a small region

user-doc/tutorials/belfast-7.txt

@@ -328,7 +328,7 @@ without the use of PT (see \ref belfast-6-exercise-5), here we can focus on the
 the PTMetaD simulation. Let's compare the behavior of the biased variable psi in the two simulations:
 
 \anchor belfast-7-ptmetadh-fig
-\image html belfast-7-ptmetadh.pdf "Time serie of the biased CV psi in a well-tempered metadynamics and PTMetaD simulations."
+\image html belfast-7-ptmetadh.png "Time serie of the biased CV psi in a well-tempered metadynamics and PTMetaD simulations."
 
 In well-tempered metadynamics (left panel), the biased variable psi looked stuck early in the simulation (t=3 ns).
 The reason was the transition of the other hidden degree of freedom phi from one free-energy basin
@@ -336,7 +336,7 @@ to the other. In the PTMetaD case (right panel), this seems not to happen. To be
 we can plot the time evolution of the hidden degree of freedom phi in the two cases.
 
 \anchor belfast-7-ptmetadhidd-fig
-\image html belfast-7-ptmetadhidd.pdf "Time serie of the CV phi in a well-tempered metadynamics and PTMetaD simulations."
+\image html belfast-7-ptmetadhidd.png "Time serie of the CV phi in a well-tempered metadynamics and PTMetaD simulations."
 
 Thanks to the excursions at high temperature, in the PTMetaD simulation the transition of the CV phi
 between the two basins is accelerate. As a result, the convergence of the reconstructed free energy in psi