lalinference_pipe_utils.py: Stop using subprocess and calculate horizon distance in readLValert. Some minor changes.
- To stop using subprocess in readLValert, use ligo.gracedb.rest to download coinc.xml and psd.xml.gz and use functions in lalsimulation to calculate duration and srate.
- Since horizon distance is not provided by gstlal, calculate horizon distance in readLValert if gstlal.reference_psd is imported successfully.
- Minor changes ( Change names of some modules since they are too generic names and change the name of ContentHandler for coinc.xml. Stop importing modules which are not used and unify the indentation rule in readLValert. )