SiPMAdc is still experimental and under discussion

[EdoPro98]

Changes proposed in this pull-request:

• Removed SiPMDigitalSignal and SiPMAdc since they are still experimental. SiPM simulation now gives a SiPMAnalogSignal that can be managed in the same way. The output is basically the same, except that integral is now a double
• Modified threshold parameter. When using analog signals a signal amplitude of 1 corresponds to 1 photoelectron. Hence the threshold has been set to 1.5 to suppress DCR
• Removed copy of SiPMAnalogSignal.waveform into a std::vector. Each sample of SiPMAnalogSignal can be accessed using [i] operator.

To be done:

• [x] Build and check if everything is working fine