Add "description" attribute to named lists

[TylerSagendorf]

Add "description" attribute to store optional descriptions for each molecular signature in a list.

• Modify readGMT to read the second entry of each row of a GMT file and save as the descriptions.
• Write the descriptions to GMT files with writeGMT.
• Modify filterSets to include the "description" attribute regardless of the class of x. The description should be subset when the list is.