import google spreadsheet year validation

[uf0]

if data__start_date and/or data__end_date are empty look at data__year

if data__year is a year (e.g. 1918):

• data__start_date -> '1918-01-01'
• data__end_date -> '1918-12-31'

if data__year is uncertain:

• data__start_date -> '1914-01-01'
• data__end_date -> '1918-12-31'

[danieleguido]

thanks @uf0, I've set the priority to enhancement