Update README.md

<details><summary>Sample graphs from powdist</summary>

For reference, we show here the two figures produced by powdist running on the first 512 seconds of the SiFAP2 3FGLJ1544_20180418_N1_Bary.fits observation with the command `[VsuEtot,epsct,hmp] = powdist_f(Nyq,tsplit,false,'grafiPS.png','grafiCT.png')`. \

For reference, we show here the two figures produced by powdist running on the first 512 seconds of the SiFAP2 3FGLJ1544_20180418_N1_Bary.fits observation with the command `powdist(Nyq,tsplit,false,'grafiPS.png','grafiCT.png')`. \

For compatibility with this readme file, they were produced as png files, but for nicer formatting we suggest to export as pdf.

<img src="sample_figures/grafiPS_3FGLJ1544_20180418_N1_Bary.fits_m1.png" />

<img src="sample_figures/grafiCT_3FGLJ1544_20180418_N1_Bary.fits_m1.png" />

<p>

    <em> Sample second graph produced by powdist, showing: </em>Top<em> - the distribution of the number of counts per bin, both in the data (crosses) and in the models calculated by the function (coloured lines); </em>Bottom<em> - the relative errors, defined as </em><math>|x^model_j - x^data_j|/x^data_j</math><em>, between any of the models and the data (points using the same colour as the lines above).</em>

    <em> Sample second graph produced by powdist, showing: </em>Top<em> - the distribution of the number of counts per bin, both in the data (crosses) and in the models calculated by the function (coloured lines); </em>Bottom<em> - the relative errors, defined as </em><math>|n^model_j - n^data_j|/n^data_j</math><em>, between any of the models and the data (points using the same colour as the lines above).</em>

</p>