Add interpolation code and examples

revert stuff
one more round  + restrict to EEG
address discussion
add utils
make_interpolation_matrix -> private
update whats new
revert change in example
ENH: Fix logic for searching files in bem plot
Merge pull request #1698 from mainakjas/fix_bemplot

ENH: Fix logic for searching files in bem plot
typos
allclose
Merge pull request #1696 from dengemann/interpolation

ENH: Add interpolation code and example
fix #1700
Merge pull request #1701 from dengemann/ylim_plot_topo

FIX #1700