Write trace co-ordinates to file

[ns-rse]

As part of the in-progress DNA Tracing refactoring (#183) the writeCoordinates() method of dnaTrace class has been removed as it was not used anywhere.

@Jean-Du has indicated it is a desirable feature to retain though.

As this is an I/O (Input/Output) operation a new function should be written when refactoring dnaTrace that can be used to write these to file.

Currently multiple np.ndarray are appended so that all grains co-ordinates are written to a single file. With the intended restructuring such that traceDNA class processes individual grains and the looping over multiple grains is handled by topostats.processing.process_scan() then it will have to return the co-ordinates (ideally as a pd.DataFrame) so that they can be concatenated and written to CSV.

[ns-rse]

#609 has left a place holder for returning a dictionary or trace co-ordinates as and the structure proposed in #605 will write these dictionaries to disk. Currently this is as a Python pickle but there may be merit/utility in writing to JSON.

This is separate to the work in progress on #601 which is looking great and breaks up the behemoth that process_scan() has become into individual functions that will make extend #517/#540.

[ns-rse]

Duplicate of #488