Cannot have multiple maps in a single grppi expression

[dmardavies]

Trying to use more than one output requires using more than one grppi expression. For example:

for(int i = 0; i < 10; i++){
        a[i] = 0;
        b[i] = 0;
}

has to be represented as:

grppi::map(grppi::dynamic_execution(), a, a, [=](auto grppi_a){return  0;});
grppi::map(grppi::dynamic_execution(), b, b, [=](auto grppi_b){return  0;});