Added comments to SequenceDictionary.mapTo.

Removed some extraneous comments from AdamContext.

Added another test to SequenceDictionarySuite, to (specifically) test the interaction
between mapTo, remap, and isCompatible, and also to check that the linear probing hash
method for resolving conflicting sequence identifiers between dictionaries is working
correctly.

When we're reading to merge, I can rebase this commit back into the main commit.