Empty room filtering

@pravsripad  I have added the filtering part in the noise covariance function,  and tested it.Please confirm the file name standardization I added. Here I have further suggestions: I think it's better to make the file name of cov matrix by the function; add parameters 'flow' and 'fhigh' in 'noise covariance function', when noise covariance function calls 'apply_filter', the bandwidth of filtering can be edited flexibility. 
```ruby
def apply_create_noise_covariance(fname_empty_room, flow=1, fhigh=45):
     ....
     apply_filter(fn_in, flow=flow, fhigh=fhigh, order=4, njobs=4)
```