apply source compression filter in hsload

[jreadey]

Currently hsload ignores the source datasets compression filter(s) and just applies whatever compression filter is given in the --compression flag.
Would seem better to use the source datasets compresssion filter if supplied. If no compression filter, use the --compression filter, and if no flag supplied, don't use compression.
Add a --nocompress flag that would have the effect of ignoring any existing filters (the current behavior)

[jreadey]

Actually filters from the source file were being applied. Added an option --ignore-filters that will only apply filters given by the --compression argument (if any).