Added PNS calculations

[FrankZijlstra]

This is a direct translation of the MATLAB Pulseq calcPNS function to python, renamed to calculate_pns. This includes a partial translation of the MATLAB safe_pns_prediction repository (https://github.com/filip-szczepankiewicz/safe_pns_prediction), put into utils/safe_pns_prediction.py. To test the functionality, this adds the pnsTest sequence "example" from MATLAB Pulseq.

A few, mostly stylistic, things may need some review:

• readasc and asc_to_hw are Siemens specific, might need to be put in a place for Siemens-specific code? (in Pulseq only readasc is separated)
• All functions from the SAFE code are in the same file, and maintain the MATLAB comments (as far as I'm concerned it really is not meant to be more than a direct translation). And what about license terms: safe_pns_prediction is BSD-3, pypulseq is AGPL-3?
• Location of the SAFE code (/utils) and pns_test.py (sequence examples)?

[btasdelen]

To address your questions:

• I agree, it is better to Siemens specific functions into its own directory. If you are positive read_asc is explicitly Siemens specific, we don't have to follow Pulseq hierarchy, we can also put it in Siemens.
• BSD is compatible with AGPL. I am not clear if it is a must, as you did modify the code significantly even though it is a translation, but we should retain the LICENSE and copyright notice for it. There are a couple of ways to do it:

1. We can paste their license file into the file safe_pns_prediction.
2. We can append our license with their license.
3. We can paste the license and explain the licensing situation in the documentation.

It is your call to pick, but 1 seems the easiest.

• I think the file positions are ok, but I would consider putting calc_pns into utils as well. Your call.

[FrankZijlstra]

I moved the Siemens functions into utils/siemens and added the BSD license into safe_pns_prediction.
I did not move calc_pns to remain consistent with MATLAB pulseq (similar as for ext_test_report, read_seq, etc.)

I updated the previous commit to keep the pull-request to a single commit.