[REF] refactored  '_price_get_multi' method

Logic factored out from method '_price_get_multi' of
product.pricelist model:
   - fetching pricelist items for specific version and products
   - checking if rule is applicable for specific product
   - calcaulation of price modification by specific rule
   - getting base price from specific rule
       - getting base price from partner section
       - getting base price from other pricelist
       - getting base price from file of product model
   - getting correct pricelist.version for specified pricelist and date
    
Reasons:
   - better structure of code.
   - To ease way other modules could extend pricelist functionalyty.