Add "mask" or similar argument to microfluidic_chip_pipe to skip circle finding step

[palmhjell]

In many experiments we want to use a mask from the final standard (e.g., the highest signal we can get) and apply that mask directly to all other standard/assay images. We can do this in post by updating the ".fg" attribute from one pipeline object with another, but it would save time to allow us to add this directly to the pipeline to skip the circle-finding step if we're going to overwrite it anyways.