Implement easy quit-out from verifications (from CAS-4146)

[Kana Sugimoto]

A request from scientists at NRAO. Implement a way to exit from per row verification. Per row verification is implemented in smooth, poly_baseline, and auto_poly_baseline in scantable class, but there's no way to exit from verification loop in the middle of scantable and users should verify all rows. It is nice to implement way to quit verification either by accepting or rejecting operations for all data in the following.

[Kana Sugimoto]

The plan is to implement something like this:

Available actions of verification [Y|n|a|r]

Y : Yes for current data (default) N : No for current data A : Accept all in the following and exit from verification R : Reject all in the following and exit from verification H or ?: help (show this message)

[Kana Sugimoto]

Done implementing a way to quit-out per row verification in:

• scantable.smooth
  
• scantable.poly_baseline
  
• scantable.auto_poly_baseline
  

A new method, scantable._get_verify_action(message, action=None), is generated to define common interface of verification.
The method

• simply returns the value of action as a string if its defined, or
  
• prints a dialog message on terminal to ask user select action and returns the user choice as a string (or show help).
  

Available actions of verification [Y|n|a|r]:

Y : Yes for current data (default)
N : No for current data
A : Accept all in the following and exit from verification
R : Reject all in the following and exit from verification
H or ?: help (show this message)