Add granule cell mechanisms (#421)

* Add three new mechanisms: `nax.mod`, `kdrmt.mod` and `kamt.mod`.
* Add new built-in math operators to `modcc`: `min`, `max`, `abs` and `exprelr`. `exprelr` is defined as the reciprocal of the 'exprel' function, exprel(x)=x/(exp(x)-1), exprel(1)=1. This function occurs frequently in HH-style mechanisms, and having a built-in operator avoids the ad hoc `vtrap` functions found in NMODL files in the wild.
* Split Arbor SIMD intrinsics support into AVX2- and AVX512-specific files.
* Add unit tests for new maths operators for C++, SIMD and CUDA implementations.