Define a custom type for the random field

[giordano]

Define a custom type for the random field

With this change we also take advantage of a new change introduced in
GaussianRandomFields v2.1.1 that allows us to reduce memory allocations when
sampling. For example, with current master:

julia> using BenchmarkTools, TDAC

julia> x = 1.:200.;

julia> y = 1.:200.;

julia> grf = @btime TDAC.init_gaussian_random_field_generator(1.0,1.0,1.0,$x,$y,0);
  18.287 ms (121 allocations: 5.50 MiB)

julia> f = zeros(40000);

julia> rnn = rand(160000);

julia> @btime TDAC.sample_gaussian_random_field!($f, $grf, $rnn);
  1.461 ms (10 allocations: 2.75 MiB)

With this change:

julia> using BenchmarkTools, TDAC

julia> x = 1.:200.;

julia> y = 1.:200.;

julia> grf = @btime TDAC.init_gaussian_random_field_generator(1.0,1.0,1.0,$x,$y,0,$true);
  18.102 ms (126 allocations: 8.25 MiB)

julia> f = zeros(40000);

julia> rnn = randn(size(grf.grf.data[1]));

julia> @btime TDAC.sample_gaussian_random_field!($f, $grf, $rnn);
  1.366 ms (3 allocations: 128 bytes)

The speed-up is modest, the major benefit is from having much less memory
allocations every time we do the sampling. The memory allocations have been
moved to the initialisation, which is however a one-off.