Migrate library issues from Jira

[maedoc]

Several open issues with the library on Jira could be migrated here:

• TVB-2842 - refactoring numerics in simulator to a reference backend
• TVB-2890 - gdist numpy incompatibility
• TVB-2892 - Numpy & Numba backends
• TVB-2893 - Model IR complete coverage
• TVB-2894 - GPU backend, e.g. cupy, pyopencl.array, or plain opencl code gen
• TVB-2895 - Parameter distribution spec
• TVB-2898 - Heun fix for 2nd order coupling
• TVB-3003 - Stimulation and non-integrated variables don't work together.

Some are blocking others, some are blocked, these dependencies should be expanded when creating the issues. For instance, to complete the model IR coverage, we should also plugin the dfun code generation and testing for that as well, in a separate issue.

Lastly, each issue when create here should link to their JIRA correspondent, resolve the JIRA issue with a backlink.