Extensions handling and multi-echo reconstruction

[beorostica]

As @cncastillo mention in a previous email: """... we are not currently handling the counters properly. I think I implemented this before Pulseq added counters LABELs as part of EXTENSIONS. It is a very good idea to read them if they are available and add them to the raw data. The closer to the scanner the better. If the counters are not set in the sequence we can do a "good guess" like we have now. But our current guess does not work with multi-echo seqs in which the acquisitions are interleaved. A patch solution would be to have an optional parameter "multiecho=true/false" to correct the counters. Regarding the k-space positions, I believe integrating should give the correct k-space positions for both echoes, unless there is something I am missing."""