177 improve annotation mapping through readgmt

[axelklenk]

This PR contains the following:

• the annotation slot in all parameter objects is no longer a simple character (containing the name of an annotation package) but a GSEABase::GeneIdentifierType object
• it can be set using the annotation argument of the parameter object's constructor (that eventually has a use)
• • if NULL it will be attempted to use the expression data set's gene identifier type (or, in other words, the gene identifier type provided at parameter object construction time overrides that of the expression data set)
• • otherwise it will be set to NullIdentifier() -- a parameter object must not contain a NULL gene identifier type
• documentation update describing the above
• more general overhaul of the documentation of parameter object classes and constructors

It does not yet contain (to be done): gene identifier mapping for container-less expression data sets (matrix and dgCMatrix) and gene sets (list of character)