ContactLogger: add celltype

Generalize the ContactLogger such that:

• it reports on all cell types if celltype is not specified (make celltype spec optional)
• include the celltype of the contacting cell
• have an option to record the duration of the cell-cell contact (store internally)

The final output should look like this:

time | cell.id | cell.type | [cell.id of neighbor] | [cell.type of neighbor] | contact_length | [contact_duration]

Also see #73.