Use `char` to store boolean values in C++ standalone. Fixes #317
Only calculate `i` in subgroups with a subexpression if there is actually a non-zero value to subtract (i.e. not for subgroups at the beginning of a group)
Fix the loading of some constant, read-only state variables (e.g. `N_incoming` in `Synapses`) in C++ standalone.
Merge pull request #319 from brian-team/bool_as_char

Bool as char