Fix syntax error in unit test for tag @slot
Fix bug in usage generation for replacement methods: 
* previous behaviour was: fun(x, y, value) <- value
* new (I think correct behaviour) is: fun(x, y) <- value
* Add support for "setReplaceMethod"
* Fix issue in signature generation for generic/methods defined in the package: for some reason the signature stored in slot @defined of MethodDefinition objects do not contain the full specification for such S4 methods. E.g:

```
setGeneric('fun', function(a, b, c) standardGeneric('fun'))
setMethod('fun', 'numeric', function(a, b, c){}) 
```
was generating signature ``(numeric)`` instead of ``(numeric,ANY,ANY)``.

* Add many unit tests to check the correctness of usage, aliases, arguments for S4 generic and methods, including replacement versions, for different kinds of calls to setMethod and setReplaceMethod.
* Fix issue with aliases in S4 classes: it can -- often -- happen that a function or generic exists with the same name as the class (e.g. a constructor method), which causes problems to the lookup of inherited parameters, which finds multiple Rd files associated with the topic.
This commit ensure that the plain name of the class is added to the alias list only if not function exists with the same name (at least within in the package).
Also default rdname (src_name) should be NAME-class, consistently with what package.skeleton produces.
* fix bug of fix a0188a263d : arguments '...' were also counted in the number of arguments, and one too many 'ANY' was append to the signature vector.
* add unit test for the case of generic with '...' in the signature
Work on inheritParams and lookup:
* improve error/warning report in inheritParam lookup, by specifying the srcref location of the problems and distinguishing between different causes of errors
* fix bug: it was crashing when no documentation is found in a tiers package
* fix bug: it was crashing when no argument section was found in a tiers package Rd file
* fix possible conflict between class and function topics: this required changing bit here and there as it touched the the partita's element src_name
I think solves Issue #82, #83.
* added a test to check for the correct export of generic functions
Revert "Work on inheritParams and lookup:"

This reverts commit 503a9c948b8bcf6f2a7f8a3b4556324d6da7f337.
I have realised that some of the changes I made are too invasive, and have a hopefully better solution coming.
Bug fixes for S4 classes:\n* fix a bug introduced with commit a0188a263df1a2c409f844e6f3e5867ef799d104: define another element  to hold data about default name, as element  is used elsewhere as the R access name for classes (error was identified running the unit tests)\n * change unit test for aliases of S4 classes, which should include the plain name even if a functions with the same name exists => will pop up R Rd selection tools.
Fix error when topic not found or with no argument doc in external packages
Add unit test for checking correct generic export directive, in relation to the fix commited in [s4 6034a9d]
Improve warning/error for @inheritParams:\n * provides srcref details on problematic origin and target chunks\n* Provides more details on problems when inheriting from generics from external packages\n* add some unit tests to check feature
patch cleanup_s4 to deal properly with classRepresentations

When dealing with classRepresentations cleanup_s4 fails in some complex cases
with interlinked classes.

I am getting the following error:

Error in .requirePackage(classDef@package) :
  unable to find required package ‘roxygen_test’

Apparently, this occurs when removeClass cannot find a class definition and
tries to re-load the package "roxygen_test".

The patch slightly modifies the order of class clean-ups,  first ordinary
classes, then classRepresentations if any.