Utilize the debug flag to output the membrane potential and current in LeakyIAF
remove float casting of edge attributes
change the data type of 'model' attribute to string in four GEXF files.
switch to LPU_rev in sensory_int_demo; use AGG as backend for its visualization
major fixes to LPU_rev; LPU.py can now be phased out once all examples are tested with LPU_rev; see details below.

1. fixed wrong logic in update buffer.
2. make sure _read_external() reads all available input.
3. fixed incorrect assumption of in_gpot_dict and in_spike_dict maintaining the same order as conn_dict.
4. do not execute neu/syn update at the first run step when the LPU is not running by itself.