Improve ch

[drizk1]

• adds @head for limiting number of collected rows
• adds support for reading URLS in db_table with ClickHouse
• adds support for reading from multiple files at once as a vector of urls in db_table when using ClickHouse
  • ie db_table(db, ["url1", "url2"])
• adds docs around using * for reading in multiple files from folder
• adds docs for db_table
• adds docs for previewing or saving intermediate tables in ongoing @chain
• Bugfix: @count updates metadata