Add a Value error is smap+extraction_mode is not macthing

[chaithyagr]

The current implementation is as follows:

    if mode == 'FFT':
           ...
    elif mode == 'NFFT':
           ...
    elif mode == 'Stack':
           ...
    else:
           ...

This might cause an unwanted run of gridding which is very slow. We need to add a value error here. Also, I just checked that the documentation is not up to date with Stack method, adding that as well.