clarify input vs control

[kelly-sovacool]

see #174 (comment)

We'll need to think about the difference between "input" and "control" in the metadata file and may want to assign a different name for each. Since we could have IgG instead of "input" this is can be confusing what this column is defining. I can trace the "input" param to macs2 -c, which is the 'control' option, making it even more confusing.