added first case study + corrected merge particle

Added tolerance to merging of particles since was (again?) an
equality...
added second case study
added third case study sketch
discovering membrane problem
found bug in cable slack test + membrane example

needed to take into account length coefficient
Merge branch 'casestudiesstart' into devel
added case study 4 as variation of cs 3

replacing cable with bent element
first critique case studies 1, 3 and 4
Merge remote-tracking branch 'origin/devel'