Remove malloc.h in Forward Modeling code

[jwreep]

In Forward_Model/source/:

forward.cpp, instrument.cpp, ion.cpp, loop.cpp, and strand.cpp each have <malloc.h> included, which will cause compilation errors on Mac or Linux (and certain compilers).

<malloc.h> can be replaced with <stdlib.h>, which should work on all operating systems.

[wtbarnes]

Yeah I've also had an issue with this as well. In my experience, it tends to be an issue on macOS when you don't have the XCode developer tools installed.

[wtbarnes]

This isn't just in the forward modeling code, but pops up in the "Radiation_Model" module as well.

[jwreep]

These should be removed in the Radiation_Model in newer versions. I think the Forward_Model is the last place they show up.

[wtbarnes]

Ah you're right. I am thinking of an older version of the Radiation Model code.