genn-team/genn: GeNN 3.1.1

This release fixes several small bugs found in GeNN 3.1.0 and implements some small features:

• Added new synapse matrix types SPARSE_GLOBALG_INDIVIDUAL_PSM, DENSE_GLOBALG_INDIVIDUAL_PSM and BITMASK_GLOBALG_INDIVIDUAL_PSM to handle case where synapses with no individual state have a postsynaptic model with state variables e.g. an alpha synapse.

• Correctly handle aliases which refer to other aliases in SpineML models
• Fixed issues with presynaptically parallelised synapse populations where the postsynaptic population is small enough for input to be accumulated in shared memory