ENH: Add plot_trans function
ENH: Moving _find_trans to transforms.py to avoid circular import
DOC: Added plot_trans to whats_new
FIX ENH: Example + addressing Alex's comments
TEST: Added test for plot_trans
DOC FIX: Adding docstring to example + making helmet transparent
ENH: Add EEG electrode display to plot_trans
FIX: More explicit checking of ch_type
drop support for picking channels if Raw not preloaded
FIX: Raise ValueError for random string as ch_type
Merge pull request #1323 from dengemann/drop_channels_preload

[BUG]: drop support for picking channels if Raw not preloaded
Merge pull request #1320 from mainakjas/plot_trans

ENH: Add plot_trans function