small fixes

- constrain hobs < 1 for weights
- fix hsq_cov
- make sure LD column names aren’t the same for weights and ref
- return a jackknife object
merge conflicts resolved
renamed lddc the module to ldscore.
2 small bug fixes
added support for gzipped .l2.ldscore files
small fix in gzip support
fixed unbiased l2 bug, added .gz to .gitignore
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merge