v0.12.0: New spike reset method

• added option to define spiking networks which contain the spike reset mechanism in their governing differential equations
• changed the name of the keyword argument spike_def of the method Network.add_diffeq_node to reset_var
• added a keyword argument reset to the Network.add_diffeq_node that controls whether the spike reset mechanism of RectiPy should be used for a given node or whether it has an intrinsic spike reset mechanism
• added a QIF neuron template that uses an intrinsic spike reset mechanism
• updated the pytest library to account for the changes