Add experimental code to fast compute gc.
delete useless code and comments in AnalyseSeriesLevinson.
AnalyseSeriesLevinson.m: now usable, faster than AnalyseSeriesFast.m for p>20. and more than 10 times faster for p=500.
A2S.m: fix the improper initialization which might impact performance.
AnalyseSeriesFast.m: make it also accept cov input.
getcovpdFile.m: fix typo in comment.
Fix Matlab compatibility problem.