sibsim4 - 0.20-5 main

The SIBsim4 project is based on sim4, which is a program designed to align
an expressed DNA sequence with a genomic sequence, allowing for introns.
SIBsim4 is a fairly extensive rewrite of the original code with the following
goals:
* speed improvement;
* allow large, chromosome scale, DNA sequences to be used;
* provide more detailed output about splice types;
* provide more detailed output about polyA sites;
* misc code cleanups and fixes.