better extraction of slit profile

[leejjoon]

Optimal spectral extraction uses a slit profile (profile of a star along the slit direction). Currently, a single slit profile is used for all orders, which is a summation (then normalized) of slit profiles extracted from central part of each orders (x pixel range between 800 ~ 1200).

This may needs to be improved as current implemenation does not account the instrumental flexure.

*) as the effect of flexure will different for different orders, slit profile will be slightly different between orders.

*) Also, the slit profile may slightly change (shift) along the wavelength direction (within a given order) due to the flexure.