Add regul_param as a property
Add regul_param as a property

This allows flushing the cached methods when it is set. This was a
problem with the Hessian, since it changed when the regul_param was
changed but the code kept using the old  cached version.

Added warning for attributes that you shouldn't change when subclassing
Misfit.

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