fcm-class: Virtual class "fcm" for a feature co-occurrence matrix
Description
The fcm class of object is a special type of dfm
  object with additional slots, described below.Slots
- context
- the context definition
windowthe size of the window, if context = "window"counthow co-occurrences are countedweightscontext weighting for distance from target feature, equal in length to windowtriwhether the lower triangle of the symmetric \(V \times V\) matrix is recordedorderedwhether a term appears before or after the target feature 
are counted seperately