resolve conflicting labels (qutip/qutip-doc#107)
resolve conflicting labels (qutip/qutip-doc#107)

Changed label master_equation to lindblad_master_equation to prevent reference to an equation already labeled in dynamics-photocurrent.rst
Also changed ^{+} to ^\dagger in equation lindblad_master_equation
cgranade/qutip,qutip/qutip,qutip/qutip,cgranade/qutip