Price calculation: in default script, return all operands and fix variable additional part

variable_additional_price was not multiplied by the result of getPricingVariable
as it was done in previous implementation.

In order that the caller has more information about how the price was computed,
Resource_getPriceCalculationOperandDict now returns the result of
getPriceParameterDict with following changed:
- list values involved in priced calculation are replaced by their sums
- 'price' key is added to holds the computed price

Also removing fallback code from Resource class, since it's been a long time
that erp5_pdm provides Resource_getPriceCalculationOperandDict
Resource_getPrice script is kept in case some unrelated code still uses it.