RIR format

[singaxiong]

Currently, the code assumes a very specific meta-data format for the RIRs. Will need to define a standard format that is flexible and easy to use.

By flexible, it should support variable number of metadata for each RIR. For example, some RIRs have information about room size, source-to-sensor distance, azimuth angle, reverberation time, etc, but some RIRs do not have any meta data. We need to be able to support both of them.

We also need to define the way to store the RIRs waveforms. One option is to store multiple multi-channel RIRs from different source positions to the same sensor(s) position(s) in one file, so we can have some of them for different speech sources, and some of them for directional noise sources.