Review and fix hardcoded values

[simleo]

The code contains several hardcoded values that we should get rid of. For instance, consider the current TrackMate reader. Both TrackMate examples currently list the following in their configuration file:

[TRACKING_DATA]
x_coord_cmso = POSITION_X
y_coord_cmso = POSITION_Y
z_coord_cmso = 
frame_cmso = FRAME
object_id_cmso = SPOT_ID
link_id_cmso = LINK_ID

Out of these, POSITION_{X,Y} and FRAME are used to look up information in the input TrackMate file, while SPOT_ID and LINK_ID are used as dataframe column headers (and eventually end up in the output .csv files). However, none of the above is actually read from the configuration file, it's all hardcoded.

See also: #8, #14.