NEMO-2630: Add model validations for ALL decimal values in Spree

done again 2-2-nemo-stable branch as a single commit
@Fortisque This line break the nemo_spree_core build. Because we have the same validation in https://github.secureserver.net/PC/nemo_spree/blob/master/core/app/models/spree/tax_rate_decorator.rb#L3

Should remove the one in nemo_spree_core