update
add notes
setup layer
port to python, save
save, the right port
update
refactor PML stretching function
fix plot xylim
refactor discretization
refactor solver Stern & add normalization
plot mode profiles
fix pep8 warning
update testing
fix pyflake warnings
move tests to source file

Thus, we do not need to import files explicitly
update docstrings
Add lone period fiber grating script.
add option to return all modes
update lpfg script
use eigs (sparse.linalg) instead of eig

eigs support generate part of eigvals, which is just I need. Save a lot of
computation resources.