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)?

[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.