Memory leak

[jessjaco]

I believe I've pinned down a memory leak that happens whenever fractional_cover._compute fractions is called iteratively within a process, either due to processing several input scenes, or using the dask processing. I believe the culprit is this pattern in fc/unmix/unmiximage_wrapper.c:

fc/fc/unmix/unmiximage_wrapper.c

Lines 102 to 104 in f2a7481

	fractionsImage_obj = (PyArrayObject*) PyArray_FromArray(fractionsImage_obj,
	PyArray_DescrFromType(NPY_DOUBLE),
	NPY_ARRAY_C_CONTIGUOUS);

It appears this operation does not free the memory held by the "original" fractionsImage_obj before pointing to a new object.

I will submit a PR for this momentarily. Happy to provide more evidence if needed.