readme update - how to fix SPI problems of some modules

If SPI does not work:

https://github.com/wollewald/ADXL345_WE#if-spi-does-not-work

Default SPI clock changed to 5 MHz

I noticed that the limit for the SPI clock is 5 MHz. I had 8 MHz as default and changed this to 5 MHz.

Examples with interrupts more reliable for ESP32

Examples with interrupts are more reliable for the ESP32 now. I added a readAndClearInterrupts() at the end of the setup() function.

2.2.3 did not work on non-ESP32 boards

In the last release I added the function:

_spi->begin(sckPin, misoPin, mosiPin, csPin);

...to enable users to change the SPI pins for ESP32 boards. This causes a compiler error on non-ESP32 boards, eventhough it's not used on these boards. I have now embraced this function by an #ifdef ESP32 clause.

Changing SPI Pins now possible

With this release, you can change the SPI pins, e.g. when using an ESP32. ADXL345_basic_data.ino shows how it works.

Corrected some data type issues

I converted some values wrongly into uint16_t instead of uint8_t. It doesn't change anything effectively, but now it's clean.

Replaced #define by constexpr

Several changes:

• Replaced #define by constexpr
• Tidied the constructors
• Replaced private by protected
• Replaced C-cast by static_cast

Various changes

Various changes:

• got rid of several global variables
• corrected typos
• use of pointers for reading the measured values
• corrected some variable type issues (there was no bug, but not best practice)

Calculation for pitch angle changed

I have changed the calculation of the pitch angle. The new method is more widely used than what I had implemented before. You find more information here, e.g.:

https://wiki.dfrobot.com/How_to_Use_a_Three-Axis_Accelerometer_for_Tilt_Sensing

SPI clock speed can now be set

I embedded the SPI transaction in SPI.beginTransaction(SPISetttings(clock, MSBFIRST, SPI_MODE3)) and SPI.endTransaction().

The clock can be set using the new setSPIClockSpeed(unsigned long) function.