Sorted out allocate methods (CIL/SIRF compatibility)

[evgueni-ovtchinnikov]

Changes in this pull request

Several allocate methods in STIR.py, Gadgetron.py and Reg.py are replaced with just one allocate in DataContainer class that does not copy data between Python and C++.

Testing performed

Related issues

Fixes #1212

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

Please read and adhere to the contribution guidelines.

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]

removing code is always excellent!