Write doc for S4 methods

[TheoPannetier]

From R CMD CHECK:

* checking for missing documentation entries ... WARNING
Undocumented S4 methods:
  generic '+' and siglist 'DemogParams,StagesParams'
  generic '+' and siglist 'DispersalParams,EmigrationParams'
  generic '+' and siglist 'DispersalParams,SettlementParams'
  generic '+' and siglist 'DispersalParams,TransferParams'
  generic '+' and siglist 'RSparams,DemogParams'
  generic '+' and siglist 'RSparams,DispersalParams'
  generic '+' and siglist 'RSparams,GeneticsParams'
  generic '+' and siglist 'RSparams,InitialisationParams'
  generic '+' and siglist 'RSparams,LandParams'
  generic '+' and siglist 'RSparams,SimulationParams'
  generic 'ColonisationStats' and siglist 'RSparams'
  generic 'ColonisationStats' and siglist 'data.frame'
  generic 'SMSpathLengths' and siglist 'RSparams,character'
  generic 'getLocalisedEquilPop' and siglist 'DemogParams'
  generic 'plotAbundance' and siglist 'RSparams'
  generic 'plotAbundance' and siglist 'data.frame'
  generic 'plotOccupancy' and siglist 'RSparams'
  generic 'plotOccupancy' and siglist 'data.frame'
  generic 'plotProbs' and siglist 'DispersalKernel'
  generic 'plotProbs' and siglist 'EmigrationParams'
  generic 'plotProbs' and siglist 'SettlementParams'
  generic 'plotProbs' and siglist 'StagesParams'
  generic 'plotProbs' and siglist 'StochMove'
  generic 'readPop' and siglist 'RSparams,character'
  generic 'readRange' and siglist 'RSparams,character'
All user-level objects in a package (including S4 classes and methods)
should have documentation entries.

Documentation needs to be written for all these methods.
See official Roxygen2 doc for guidance.