Avoid using import * in Python MR and PET demo scripts

[evgueni-ovtchinnikov]

Changes in this pull request

Currently, we use

exec('from sirf.' + args['--engine'] + ' import *')

to import reconstruction modules (engines) for MR (Gadgetron) and PET (STIR). This practice is not recommended, and raises Codacy complaints.

This PR suggests using importlib function instead, e.g. for MR:

import importlib
mr = importlib.import_module('sirf.' + args['--engine'])
acq_data = mr.AcquisitionData()

Testing performed

The demo scripts run ok on my VM.

Related issues

Checklist before requesting a review

• I have performed a self-review of my code
• I have added docstrings/doxygen in line with the guidance in the developer guide
• I have implemented unit tests that cover any new or modified functionality
• The code builds and runs on my machine
• CHANGES.md has been updated with any functionality change

Contribution Notes

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Please tick the following:

• The content of this Pull Request (the Contribution) is intentionally submitted for inclusion in SIRF (the Work) under the terms and conditions of the Apache-2.0 License.

[KrisThielemans]

great idea.

[KrisThielemans]

looks good. 2 comments/questions.