Read only vars

[neworderofjamie]

This change allows you to specify whether state variables are readonly when you define a model e.g. in the ``IzhikevichVariable (using syntax similar to that suggested in #55):

class IzhikevichVariable : public Izhikevich
{
public:
    DECLARE_MODEL(NeuronModels::IzhikevichVariable, 0, 6);

    SET_PARAM_NAMES({});
    SET_VARS({{"V","scalar"}, {"U", "scalar"}, 
              {"a", "scalar", VarAccess::READ_ONLY}, {"b", "scalar", VarAccess::READ_ONLY}, 
              {"c", "scalar", VarAccess::READ_ONLY}, {"d", "scalar", VarAccess::READ_ONLY}});
};

Knowing whether state variables are read-only is useful in two ways:

• Immediately: to remove unnecessary writes of neuron and postsynaptic; and pre and postsynaptic post weight update model state variable back to global memory (to some extent aiding Refine global - register -global transfers #55) - depending on dumbness of CUDA compiler this could reduce memory bandwidth and reduce register pressure
• In future: this allows all kinds of useful 'analysis' e.g. to determine if state variables can be optimised out either in the style of GLOBALG
  (this is something that I think is done in the PyNN, SpineML and, I suspect, Brian frontends) or by the long-dreamed-of Izhikevich style re-sampling

[codecov]

Codecov Report

Merging #247 into master will increase coverage by 0.02%.
The diff coverage is 86.76%.

@@            Coverage Diff             @@
##           master     #247      +/-   ##
==========================================
+ Coverage   85.42%   85.44%   +0.02%     
==========================================
  Files          45       45              
  Lines        6319     6342      +23     
==========================================
+ Hits         5398     5419      +21     
- Misses        921      923       +2

Impacted Files	Coverage Δ
include/genn/genn/neuronModels.h	80% <ø> (ø)	⬆️
include/genn/genn/code_generator/codeGenUtils.h	95.45% <ø> (ø)	⬆️
include/genn/genn/weightUpdateModels.h	43.75% <0%> (ø)	⬆️
.../genn/genn/code_generator/generateSynapseUpdate.cc	100% <100%> (ø)	⬆️
src/genn/genn/code_generator/codeGenUtils.cc	93.06% <100%> (ø)	⬆️
src/genn/backends/cuda/backend.cc	88.22% <100%> (ø)	⬆️
...c/genn/genn/code_generator/generateNeuronUpdate.cc	96.98% <100%> (+0.13%)	⬆️
src/genn/genn/modelSpec.cc	63.1% <100%> (ø)	⬆️
src/genn/genn/code_generator/generateInit.cc	97.71% <100%> (ø)	⬆️
include/genn/genn/initSparseConnectivitySnippet.h	40% <33.33%> (ø)	⬆️
... and 2 more

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