Better control the number of GRanges

[lldelisle]

This PR does mainly 2 things:

• Adds an option to force FeatureMatrix to return as many rows as input granges (very useful when you have peaks on some chromosomes which are not covered in all samples).
• Fixes CreateMotifMatrix so it always get the good number of rows. This also implies:
  • Modification of an example
  • Enable cell to be null in AddMissing to only modify features.

I added tests but tests are limited as motifmatchr is not a dependency.