Revise, cleanup and document the entire digitization strategy

[lucabaldini]

This is a meta-issue to keep track of the basic tasks involved in the overall revamping of the digitization.

Basically I think we want to converge on two basic readout options (circular and rectangular), get rid of the sparse readout, clean up and rationalize the interfaces and properly document the entire thing. This is a long-ish-term project, involving several different issues.

• issue Implement a DigiCircularReadout variant where the geometrical ordering of the 6 lowest pixels is fixed #80
• issue Remove the DigiEventSparse readout #79
• issue DigiOutputFiles created without readoutmode attribute #56
• issue DigiEventSparse reconstruction has no physical sense #52
• issue Change the signature of the read() functions in the readout structures  #51
• issue Close the loop over the different types of digitizaiton #50

I think we can attack the issue of restructuring the event files after we have done this.