Counter: expand options for strict interval matching

[AlexTate]

The following are valid options for the Match column of the Features Sheet:

• Partial: single base overlap for a match
• Full: (feature start ≤ alignment interval ≤ feature end) for a match

The following options should be added to this selector:

• Exact: (feature start = alignment start; feature end = alignment end) for a match
• 5' anchored:
  • If same strand: (feature start = alignment start) for a match
  • If opposite strand: (feature start = alignment end) for a match
• 3' anchored:
  • If same strand: (feature end = alignment end) for a match
  • If opposite strand: (feature end = alignment start) for a match

[AlexTate]

The above description could lead to some confusion. The strand dependency of the 5' and 3' anchored selectors is not a consideration when processing GFF and SAM data since all positions are defined using the same coordinate system and origin regardless of strand (that is, the start position in these records does not imply 5' position)