Started the base Objective class
Started the base Objective class

This will be the base class for every data-misfit and regularization. It
interfaces with the solvers and overloads operators. So far I have just
the basic skeleton.

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