Loads expression data.
load.expression(path, variable = NULL, entries = NULL)A path of a file that contains attribute data to be loaded.
When the file is a loadable .Rdata,
name of the dataframe object in the .Rdata
(that actually contains the relevant data)
has to be specified as well.
An optional string that designates the name of a
dataframe object that has been saved in an .Rdata.
(See the description of path.)
An optional string array that designates the entries of the dataframe to be loaded.
An object that represents the expression data of microarray/RNA-seq.
    Internally, the object holds a matrix of size
    nsamples * ngenes.