bugfixes and optimizations for data viewer. Fixes BZDNG-252

examples/code_fragments/img/create_split_image.py

examples/code_fragments/img/fft_li.py

examples/code_fragments/img/modulate_image.py

examples/code_fragments/img/spoke_pattern.py

examples/code_fragments/img/view_phase_diff.py

modules/gui/src/data_viewer/data_viewer.cc

modules/gui/src/data_viewer/data_viewer.hh

modules/gui/src/data_viewer/data_viewer_panel_base.cc

modules/gui/src/data_viewer/data_viewer_panel_base.hh

modules/gui/src/data_viewer/fft_panel.cc

modules/gui/src/file_loader.cc

modules/gui/src/gosty_app.cc

modules/gui/src/scene_selection.cc