Add thread safety, robustness, and filesystem tests

[marcuschangarm]

• Use mutex to ensure each operation is atomic.
• Add explicit software reset during initialization.
• Add filesystem tests.
• Updated documentation.

[marcuschangarm]

I didn't use the SPI mutex after all since that would lock the SPI bus longer than necessary.

The not-write-protect pin comes after the frequency so that the arguments are compatible with the spif-driver.

The software reset is there because the device seem to enter an undefined state sometimes.