Added Arduino compliant out of Range handling for the at() function

[AlexanderTonn]

Beforehand, I have to say that I appreciate it that you've created this library! 👍

The big advantage of at() in the std::array lib is the detection of out of ranges indices. The function is throwing an exception if the inserted index is out of range of the array.

• I added a small handling if an invalid index is passed to at(). If the UART Interface is started, the user will get an error message.
• however in case of out of range the function will return the value of the last valid index
• I also added an examples .ino file for showing the at() function with different data types.
• tested with arduino MEGA 2560 R3

[AlexanderTonn]

I would remove the local (not good named) variable and inserting the subtraction directly in the bracket operator

- auto outOfRangeIndex = arraySize-1;
- return values_[outOfRangeIndex]; 
+ return values_[arraySize-1]; 

[AlexanderTonn]

I would remove the local (not good named) variable and inserting the subtraction directly in the bracket operator

- auto outOfRangeIndex = arraySize-1;
- return values_[outOfRangeIndex]; 
+ return values_[arraySize-1];