Issues with integrating with units library

[Forrest-SOEffects]

This might be useful to others.
When integrating this library with https://github.com/nholthaus/units with the following setup

• platformio @ 6.1.7
• Teensy 4.1 (4.17.0)
• framework-arduinoteensy @ 1.157.220801 (1.57)
• tool-teensy @ 1.158.0 (1.58)
• toolchain-gccarmnoneeabi @ 1.50401.190816 (5.4.1)

Due to the units library requiring you to undefine certain symbols, that then causes the Eigen/Core complex.h include to fail.

I fixed this by redefining those symbols right above #include <complex.h> within Eigen/Core to satisfy its build needs.

#define	_U	01
#define	_L	02
#define	_N	04
#define	_S	010
#define _P	020
#define _C	040
#define _X	0100
#define	_B	0200

Is what I had to define.

These values were pulled from toolchain-gccarmnoneeabi/arm-none-eabi/include/ctypes.h

I don't know if this will cause some other funky issues. But I am yet to find out.