PythonAPI: create workspace method to get multi-dim Numpy array with all spectra

[mdoucet]

Although all data manipulation for analysis should be done by algorithms, there is a need for UI widgets to be able to access the data. Right now, it is only possible to use the readX/Y/E methods to get a single spectrum. In cases like SANS, where the each pixel of the 2D detector is a spectra, one would have to loop over N*N pixels to get the whole detector array. This can be very slow for a widget coded in python. We should add three methods along the lines of read_all_spectraX/Y/E, which would return a Numpy array N_x * N_y, where N_x is the number of X bins (TOF) and N_y is the number of spectra.

[mdoucet]

This issue was originally trac ticket 1778