Operator overloads for Objective
Operator overloads for Objective

Added the overloads for sum, multiply by scalar and increment (+=).
All of these produce MultiObjective instances.

fatiando/fatiando,eusoubrasileiro/fatiando_seismic,eusoubrasileiro/fatiando,eusoubrasileiro/fatiando,drandykass/fatiando,santis19/fatiando,mtb-za/fatiando,victortxa/fatiando,eusoubrasileiro/fatiando,fatiando/fatiando,victortxa/fatiando,eusoubrasileiro/fatiando_seismic,drandykass/fatiando,rafaelmds/fatiando,cmeessen/fatiando,santis19/fatiando,cmeessen/fatiando,mtb-za/fatiando,rafaelmds/fatiando