Avoid malloc where unnecessary

[willGraham01]

Running TDMS through Valgrind shows that we have a number of memory leaks from random malloc calls, and plhs never being unassigned. We also have a number of malloc'd blocks of size 0 bytes which we then write to 😬

We need to go through the codebase and check that we are not abusing the use of malloc: there are a number of cases where malloc is used when a scoped declaration would be sufficient; that is

int dims[2] = {1, 1};

rather than

int *dims = (int *) malloc(sizeof(int) * 2);
dims[0] = 1; dims[1] = 1;
free(dims);

In the case of variable-length arrays we can switch to std::vector:

std::vector<double> an(N);

as opposed to

double *an = (double *) malloc(sizeof(double) * N);