magstatics build for ARM architectures (e.g. MacOS)

[berianjames]

This PR (re-?)adds the ability to build and use MagTense on ARM architectures (i.e. any current or future MacOS). Presently only the magstatics module can be used. We can discuss whether it's appropriate to actually merge this now.

To develop a ARM build consistent with Windows / Linux, we need to:

• Identify a replacement for Intel MKL library interfaces for the micromagnetics module. The underlying libraries (BLAS etc.) are already present on MacOS through Apple's Accelerate.framework. Open question: in principle we can change the interfaces used in the f90 L-L code to a non-proprietary interface, but in practice we may need separate Fortran code.
• Similarly, this is implemented by simply adding a separate Makefile & IO module f90 file. Better if we can have a unified Makefile.

[spollok]

I have looked quickly through the files, and they seem to look fine. And I guess @berianjames has tested it successfully on an ARM architecture. As mentioned, the files are mostly a copy of the existing files. A suggestion could be trying to make a unified Makefile already. The underlying machine hardware can be inferred with UNAME_M:= $(shell uname -m) on unix-like systems and for excluding the micromagnetic part in the IO file with could use preprocessor directives, e.g., ARCH_ARM, to switch between the two cases.

[spollok]

@berianjames I have made a commit on the arch_arm64 branch for a first attempt of unification. You might want to test it on ARM architecture :)