define more intuitive error messages for frequently encountered problems
*[x] createNetwork: ncol = nrow
*[x] createEdgelist: matrix with two columns as input, values in column 1 starting with 1 and ending with total number of different values
*[x] createNodeVariable: input data should be a vector
*[x] in all "create" functions: check for missing values, all values classed as numeric (except createNodeSet)
*[x] createProcessState: check whether all input objects are in correct format/class, same number of observations, correct names => define checkProcessState function (hidden)
define more intuitive error messages for frequently encountered problems