Reghdfe Versions Save

Fix ivreghdfe bug when clustering with string variables (#276)

• Update ftools dependency to prevent parallel bug in macOS

• Fix typos in help file
• (Try yet again to) fix zenodo metadata

First non-beta version of v6

This archives the 5.9 release of reghdfe (last release of v5) prior to merging v6 from the dev branch.

First release using versioning

• No open bugs
• All tests pass
• No experimental code inside

>> download zipped package <<