allow for execution without a control

[slsevilla]

Describe what enhancement you'd like to see.
We currently allow for MACS2 to run without a control - may have situations were a control is not available or it failed sequencing. Valuable data can still be obtained for such a scenario. Because of this, an adjustment should be made to allow for running without a control

[epehrsson]

When needed, we run the pipeline by including a dummy IgG control in the sample matrix (as a sample entry = row and listed as the control for each sample). It is included in the MultiQC report and the DeepTools plots, but otherwise output is separate from that of the true samples.