Add trellis for DC

Add option to apply trellis quantization to the DC coefficients ( see
#57 ). May need further refinement to make sure block order during
trellis optimization matches order during coding.
Add option to split DC scans

Command line option -split-dc-scan is added to code DC scans
independently (instead of interleaved). It should be determined whether
this option introduces any decoder compatibility issues ( see #83 )
Option -multidcscan is renamed to -opt-dc-scan
Fix issue with DC trellis
Fix issue with scan printout

Incorrect values of scan parameter Al were printed out in verbose mode
Merge branch 'dctrellis'