Properly handle "unstructured data" from Calliope Mini

[rzbrk]

CalliopEO.py currently needs "line-oriented" data from the Calliope Mini. When Calliope Mini transmits "unstructured" data without newlines, CalliopEO.py will not continue processing the data (until it eventually receives a newline).

This could be improved by reading abitrary chunks of data from the serial input buffer. Later, CalliopEO.py can extract the lines and do further processing like adding the time stamp and so on.

Maybe, we can define this as a feature for a v2.

[rzbrk]

In my fork I created an issue branch. There, I completely rewrote the function readSerialData() (see here) to handle "unstructured data", in particular a stream of data not containing newlines or very, very long lines.

I defined a serial data buffer (see here) and new global variable MAX_LINE_LENGTH (see here). If the buffer length exceeds MAX_LINE_LENGTH the buffer is "cropped" by extracting MAX_LINE_LENGTH characters as a line which will be outputted to the file.

The functions waitSerialStart() and waitSerialEnd() are now incorporated in readSerialData().

The return value of this function is no longer a list of lines. The data is directly written to file (via function write2File()). The return value of readSerialData() is now the number of characters written to the file.

Before merging this change to main, this needs further testing. I will also adding a dedicated testcase with a hex file outputting very long lines.

[rzbrk]

Can be closed if #73 is merged to main.