Renamed stl_vector_tools.hpp --> std_vector_tools.hpp, #983
Removed Sparsity::null(), GenericMatrix::null(), replaced with isEmpty(true), #983
Refactored MX::create, #380
Added ZeroByZero MX node (singleton class), #380
Enabled use of ZeroByZero node, #380
Added MX(int,int) and MX::isNullE() for non-disruptive implementation of #380
Changed MX::isNull() --> MX::isNullE() inside MXFunctionInternal, #380
Changed MX::isNull() --> MX::isNullE() inside FXInternal, #380
Changed MX::isNull() --> MX::isNullE() inside ImplicitFunctionInternal, #380
Added Matrix::isNullE(), #380
Changed MX::isNull() --> MX::isNullE() inside XFunctionInternal, #380
MX() --> MX(0,0) inside MXFunctionInternal, #380
Replacing MX() --> MX(0,0), #380
Replacing MX() --> MX(0,0) (2), #380