Add abstraction for FRAM ring array

[PatrickKa]

Description

We need this common functionality for #179 and #204, so I made a separate issue for it.

A ring array is an array with periodic boundary conditions. Appending an element to a full ring array should cause the oldest element to be lost. Apart from the actual data, also the begin and end indexes should be stored on the FRAM. The PersistentVariables class should be used for that, which means that the indexes are also error corrected.

As mentioned in #294 we also need a fallback mechanism in case the FRAM does not work. For this, an etl::circular_buffer should be used to cache the values in RAM. The size of this circular buffer should be specified separately, as it is usually much smaller.