Implement improved detector map representation

Currently, the detector map implementation is such that it internally stores the wavelength solution of individual fibers as splines.

Sometimes a detector map needs to be adjusted (x/y shifts, rotations, focus), and regenerating those internal splines for each adjustment could lose accuracy.

While such an internal representation may usable even for the start of commissioning, an improved internal sparse representation of the detector map that handles changes (shifts, rotations and focus) in a more accurate way, should be investigated.