Low pass filter_freq and IFO.maximum_frequency discrepancy

I'm running on frame data that goes up to 2048Hz, so I set my sampling frequency to be 4096Hz so that my max frequency would be 2048. However, the low pass filter in gw.utils.process_strain_data() defaults to have a maximum frequency of 1024, so there's a drop-off in my data at that frequency, but it still goes up to 2048. I'd suggest changing the filter_freq to coincide with the IFO.maximum_frequncy or if you want to overwrite the IFO.maximum_frequency to match the filter_freq, print a warning that the user's maximum frequency is being overwritten, and that they should set the filter_freq by hand to match their desired maximum frequency. @gregory.ashton do you want to add this to the clean up detector merge request? @colm.talbot