improvements for sheet name/index checking

[jmbarbone]

add private and public methods to

• self$validate_sheet() to make wb_validate_sheet() a wrapper
• self$validate_new_sheet() to check against current sheets
• private$get_sheet() to be more strict/possibly replace self$validate_sheet()
• clean_worksheet_name() to wrap/replace replaceIllegalCharacters()

Meant to:

• increase functionality of checks
• restrict illegal characters (errors)
• restrict name nchar levels (errors)
• limit silent modification