Memory usage optimization

[Ciusss89]

1. files energy_meter/main.c and energy_meter/core.h
2. In order to optimize the system resource, we have to reduce the memory required by tread sampling and logging.

char em_sampling_stack[THREAD_STACKSIZE_SMALL];
char em_logging_stack[THREAD_STACKSIZE_LARGE];

3. Instead, to use a type double for the struct we can use two uint8_t to store the decimal and the unit part

/* @em_realtime contains all notable datas:
 * -rms_c/v▸     : real time values, they're update each sec
 * -rms_*_1m▸    : last minute average, they're update each 60 sec
 * -log_1m_ready : true when last minute average is ready
 */
struct em_realtime {
▸       double rms_c, rms_v;
▸       double rms_c_1m, rms_v_1m;
▸       bool log_1m_ready;
};

/* @em_loggin contains all temporary datas
 */
struct em_loggin {
▸       double c[60], v[60];
};