Passing SPI objects did not work correctly

Passing SPI objects did not work correctly. The reason was that I still one SPI.transfer() command in the code instead of _spi->transfer().

Bug fix in setIntVRef()

A wrong source was chosen in that function.

Replaced #define by constexpr

Various changes:

• Replaced #define by constexpr
• Replaced C-cast by static_cast
• Emptied constructor
• Replaced private by protected

Something went wrong with last release

No change vs. last release.

Added links to the corresponding tutorials on my website

No change to the code. I just added links to my website:

1. An article about the ADS1220 and the use of this library
   https://wolles-elektronikkiste.de/ads1220-4-kanal-24-bit-a-d-wandler (German)
   https://wolles-elektronikkiste.de/en/4-channel-24-bit-adc-ads1220 (English)

2. An article how to use the ADS1220 for typical applications, such as thermocouples,
   NTCs, RTDs or Wheatstone bridges:
   https://wolles-elektronikkiste.de/ads1220-teil-2-anwendungen (German)
   https://wolles-elektronikkiste.de/en/ads1220-part-2-applications (English)

PGA bypass bug fixed

PGA bypass was not visible using isPGABypassed(). This is fixed.

Something went wrong with the last release

This one should work now. It seems Arduino does not like if releases are deleted.

Example sketch was not visible in Arduino IDE

One of the example sketch was not named like the folder. Therefor the sketch didn't appear in the Arduino IDE.

Automatic PGA bypass

Main change: I have changed the code slightly to ensure that PGA is bypassed if a measurement with AVSS as negative input is done.

Bug fixed in setGain

Bug fixed in setGain