ERC20Lockable - inconsistent locking status
`Vega_Token.is_tradable()` will incorrectly return `false` if the token is never manually unlocked by the owner but `unlock_time` has passed, which will automatically unlock trading.\\n```\\n/\\*\\*\\n \\* @dev locked status, only applicable before unlock\\_date\\n \\*/\\nbool public \\_is\\_locked = true;\\n\\n/\\*\\*\\n \\* @dev Modifier that only allows function to run if either token is unlocked or time has expired.\\n \\* Throws if called while token is locked.\\n \\*/\\nmodifier onlyUnlocked() {\\n    require(!\\_is\\_locked || now > unlock\\_date);\\n    \\_;\\n}\\n\\n/\\*\\*\\n \\* @dev Internal function that unlocks token. Can only be ran before expiration (give that it's irrelevant after)\\n \\*/\\nfunction \\_unlock() internal {\\n    require(now <= unlock\\_date);\\n    \\_is\\_locked = false;\\n```\\n
declare `_is_locked` as `private` instead of `public`\\ncreate a getter method that correctly returns the locking status\\n```\\nfunction \\_isLocked() internal view {\\n       return !\\_is\\_locked || now > unlock\\_date;\\n}\\n```\\n\\nmake `modifier onlyUnlocked()` use the newly created getter (_isLocked())\\nmake `Vega_Token.is_tradeable()` use the newly created getter (_isLocked())\\n`_unlock()` should raise an errorcondition when called on an already unlocked contract\\nit could make sense to emit a “contract hast been unlocked” event for auditing purposes
```\\n/\\*\\*\\n \\* @dev locked status, only applicable before unlock\\_date\\n \\*/\\nbool public \\_is\\_locked = true;\\n\\n/\\*\\*\\n \\* @dev Modifier that only allows function to run if either token is unlocked or time has expired.\\n \\* Throws if called while token is locked.\\n \\*/\\nmodifier onlyUnlocked() {\\n    require(!\\_is\\_locked || now > unlock\\_date);\\n    \\_;\\n}\\n\\n/\\*\\*\\n \\* @dev Internal function that unlocks token. Can only be ran before expiration (give that it's irrelevant after)\\n \\*/\\nfunction \\_unlock() internal {\\n    require(now <= unlock\\_date);\\n    \\_is\\_locked = false;\\n```\\n