Fix crashing the SPD3303X processor when saving

[Riley-King]

The command *SAV <X> takes ~50 ms on the SPD3303X but we only wait for ~21 ms before trying to use it which causes it to crash and needs to be power cycled before it will work again.

Changes;

• Added a slow write function.
• save.* now uses the slow write function.

[Riley-King]

@Jasper-Harvey0 The *WAI command can not be used as the SPD3303X has not implemented it.

SPD3303X manual

[Riley-King]

Changed to using lambda+kwarg to introduce the delay.
The delay for saving groups was expanded to 100 ms because these pps' are crap and was not consistently passing tests.

1 failed, 29 passed, 4 xfailed, 1 warning in 41.51s
The failure is a result of tolerance issues, not any fault of the driver;
FAILED test/drivers/test_siglent_spd_3303X.py::test_measure_voltage[2.2-channel2-CH2] - assert 2.212 == 2.2 ± 0.01

[clint-lawrence]

The failure is a result of tolerance issues, not any fault of the driver;
FAILED test/drivers/test_siglent_spd_3303X.py::test_measure_voltage[2.2-channel2-CH2] - assert 2.212 == 2.2 ± 0.01

That tolerance is tighter than the datasheet spec for the instrument, so make sense to adjust it. +/- 50 mV is probably a reasonable spot to leave it