Added txt documents that prestudy for the detailed design of the SDP firmware.

applications/lofar2/doc/lofar_station_firmware_model.vsd

applications/lofar2/doc/prestudy/desp_hdl_design_article.txt

applications/lofar2/doc/prestudy/dupllo_aad_erko.txt

...r2/doc/prestudy/dupllo_oversampled_subband_filterbank.txt

applications/lofar2/doc/prestudy/station2_opc_ua.txt

applications/lofar2/doc/prestudy/station2_sdp_dsp.txt

...ions/lofar2/doc/prestudy/station2_sdp_firmware_design.txt

...ns/lofar2/doc/prestudy/station2_sdp_firmware_planning.txt

...tions/lofar2/doc/prestudy/station2_sdp_hdl_components.txt

applications/lofar2/doc/prestudy/station2_sdp_icd.txt

applications/lofar2/doc/prestudy/station2_sdp_m_and_c.txt

applications/lofar2/doc/prestudy/station2_sdp_ring.txt

applications/lofar2/doc/prestudy/station2_sdp_timing.txt

applications/lofar2/doc/prestudy/station2_semi_float32.txt

applications/lofar2/doc/prestudy/station2_to_do_erko.txt