Add B_s0 (B0)->KS0KS0 HLT2 line

The MR adds three HLT2 lines, selecting B_s0, B0->KS0KS0 in three different channels:

• LL where both KS0 are reconstructed with Long tracks
• LD where one is reconstructed with Long tracks and the other with Downstream tracks
• DD where both KS0 are reconstructed with Downstream tracks

Rate of lines computed over upgrade_minbias_hlt1_filtered sample is:

Hlt2BnoC_BdsToKsKsDDDDDecision  Incl: 0.0 +/- 0.0 kHz,         Excl: 0.0 +/- 0.0 kHz
(<0.067 kHz given by acceptance of 1/15k events present in the sample)
Hlt2BnoC_BdsToKsKsLLDDDecision  Incl: 0.46719 +/- 0.1765 kHz,   Excl: 0.40045 +/- 0.1634 kHz
Hlt2BnoC_BdsToKsKsLLLLDecision  Incl: 0.06674 +/- 0.0667 kHz,  Excl: 0.06674 +/- 0.0667 kHz

Efficiency over B_s0->KSKS events (evt type 13104101):

Hlt2BnoC_BdsToKsKsDDDDDecision            93/30000      eff = 0.31 +/- 0.03 %
Hlt2BnoC_BdsToKsKsLLDDDecision            104/30000     eff = 0.35 +/- 0.03 %
Hlt2BnoC_BdsToKsKsLLLLDecision            31/30000      eff = 0.10 +/- 0.02 %

FYI @yamhis @shollitt