Update figure names with underscore in simple example.

A refers to geoloop's semi-analytical depth-dependent model ,  T to inlet Temperature mode, and 

SR and MC to single run or monte carlo outcomes. see [Manual](../../manual/cli.md) for more detailed explanation.

![Fig. 2](images/SU_100_A_T_SRtimeplot_Q_b.png)

![Fig. 2](images/SU_100_A_T_SR_timeplot_Q_b.png)

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Fig. 2 Evolution of power extracted from the subsurface by the simulated single U-tube during simulated time.

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![Fig. 3](images/SU_100_A_T_SRtimeplot_T_bave_T_fo.png)

![Fig. 3](images/SU_100_A_T_SR_timeplot_T_bave_T_fo.png)

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Fig. 3 Evolution of outlet, and average borehole wall temperature in the simulated single U-tube during simulated time.