Confusing and user-unfriendly syntax for getting standard continuum

[cylammarco]

The standard continuum can only be computed in the flux calibration stage at the moment. While the science continuum can be tinkered with get_continuum().

The continuum finding of the standard target should be removed from flux calibration and handled at the onedspec level.

Computing the telluric profile should be an independent process from flux calibration.