Add support for nested inputs and outputs in WPS4R

[nuest]

The WPS 2.0 standards supports nesting of inputs and outputs. Once this is implemented in the WPS internal API, this can be mapped to groups of inputs and outputs in WPS4R annotations as follows:

# wps.in: id = sampling/size;
samplingSize <- 100
# wps.in: id = sampling/method;
method <- "default"
# wps.in: id = setttings/logoutput;

The id of the field can be split at / into group and parameter name, the structure within the script is not nested but only simple names.