Update figure names with underscore in PYG benchmark example.

The curves from the semi-analytical depth-dependent model in Geoloop and the standard implementation of g-functions are in perfect

agreement in the plots.

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

![Fig. 3](images/PYG_SU_P_P_SR_ANALYTICAL_SU_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 Geoloop benchmark simulations of the single U-tube borehole design.