Fix colormap generation and output folder name

[a-pru]

(1) If there is only a single track (len(tracks)=1) in an image sequence, writing the output images fails due to issues with the generated colormap

ERROR - test_tracktor - Failed after 0:00:46!
Traceback (most recent calls WITHOUT Sacred internals):
  File "experiments/scripts/test_tracktor.py", line 165, in main
    write_images)
  File "/databricks/driver/tracking_wo_bnw/src/tracktor/utils.py", line 200, in plot_sequence
    color=cmap(track_id)
  File "/databricks/conda/envs/databricks-ml-gpu/lib/python3.7/site-packages/matplotlib/colors.py", line 612, in __call__
    self._init()
  File "/databricks/conda/envs/databricks-ml-gpu/lib/python3.7/site-packages/matplotlib/colors.py", line 899, in _init
    self.N, self._segmentdata['red'], self._gamma)
  File "/databricks/conda/envs/databricks-ml-gpu/lib/python3.7/site-packages/matplotlib/colors.py", line 516, in _create_lookup_table
    "data mapping points must start with x=0 and end with x=1")
ValueError: data mapping points must start with x=0 and end with x=1

Generating the colormap for len(tracks)+1 labels solves this problem.

(2) In test_tracktor.py the dataset variable is a string at first, then a Factory object is assigned to this variable.
Hence, the output folders for plot_sequence() are named as <object Factory at 0xsomething> - fixed by using an intermediate variable.