Adding specifying `cosmicx` parameters in `.yaml` files.

In the `metadata` branch, `cosmicx` parameters for each detector (WFPC2, UVIS, IR, WFC, HRC, SBC) can now be specified in separate `.yaml` files, inside a new directory `MTPipeline/cosmicx_cfg`. 

To support this, two new functions have been added. 

`get_metadata()` has been added to `run_imaging_pipeline.py`. It's job is to extract all information from the headers of our input `fits` files, so that we can configure the pipeline to those files needs. At the moment, the only information it returns (as a dictionary, with a single key and value) is the detector. However, we also might want to be able to pass the readnoise and gain of the files to `cosmicx`, so we may eventually add that capability once Ticket #115 has been resolved. Additional information might be needed for the `astrodrizzle` step. At the moment, however, `get_metadata()` is only called when doing the CR rejection step, so its position in `run_imaging_pipeline` will need to be changed when what it returns is useful for the other steps.

`get_cosmicx_params()` has been added to `run_cosmicx.py`. Its job is to use `PyYaml` to read in the appropriate `cosmicx` configuration file for a given detector. The keys of this dictionary are integers, corresponding to individual FITS extensions. The entires for each integer key are themselves dictionaries, which contain the `cosmicx` parameters appropriate for that extension.