Simd partition by constraint (#494)

Changes have been made to the simd implementation of mechansim functions: 

- The node_index array (array of indices that specifies for each mechanism the CVs where it is present), is now partitioned into 4 arrays according to the constraint on each simd_vector in node_index:
    1. contiguous array: contains the indices of all simd_vectors in node_index where the elements in simd_vector are contiguous
    2. constant array: contains the indices of all simd_vectors in node_index where the elements in simd_vector are identical
    3. independent array: contains the indices of all simd_vectors in node_index where the elements in simd_vector are independent (no repetitions) but not contiguous 
    4. none array: contains the indices of all simd_vectors in node_index where the none of the above constraints apply

    When mechanism functions are executed, they loop over each of the 4 arrays separately. This allows for optimizations in every category. 

- The modcc compiler was modified to generate code for the previous changes, including the optimizations per constraint:
    1. contiguous array: we use vector load/store and vector arithmetic. 
    2. constant array: we load only one element and broadcast it into a simd_vector; we use vector arithmetic; we reduce the result; we store one element.   
    3. indepndent array: we use vector scatter/gather and vector arithmetic. 
    4. none array: we cannot operate on the simd_vector in parallel, we loop over the elements to read, perform arithmetic and write back 

- Added a mechanism benchmark for pas, hh and expsyn

- Moved/modified some functions in simd.hpp to ensure that the correct implementation of a function is being called.