Replaced __repr__s in inversion.base
Replaced __repr__s in inversion.base

Using simpler reprs in base classes since they are only for testing
purposes.

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