Update figure names with underscore in PYG benchmark example.

the two models. This is caused by model the set-up with multiple depth-segments in the Geoloop model, and is also reflected

in the calculated temperature field inside the borehole, as shown in Fig. 5 and 6. 

![Fig. 3](images/PYG_DU_P_P_SR_ANALYTICAL_DU_A_P_SRtimeplot_T_bave_T_fi_T_fo.png)

![Fig. 3](images/PYG_DU_P_P_SR_ANALYTICAL_DU_A_P_SR_timeplot_T_bave_T_fi_T_fo.png)

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Fig. 3: Timeseries plot of inlet ($T_{in}$), outlet ($T_{out}$) and average borehole wall temperature ($T_{b,ave}$) for the