Add time-dependent noise handling inside C implementation of Elens beam element

[carlidel]

Description

As HEL require fast testing with different noise patterns, it is of interest to implement directly in the C code the possibility of handling a time-dependent noise array, representing different realization/operation patterns of HEL.

The code simply implements a float array in the xobjects buffer that the C code then reads depending on the particle at_turn value. Different modes for handling out-of-lenght indexes have been implemented.

A consistency check has been performed. If you think it is of interest, I can include it in the test files.

I have updated the HEL docstring accordingly.

Checklist

Mandatory:

• [ X ] I have added tests to cover my changes
• [ X ] All the tests are passing, including my new ones
• [ X ] I described my changes in this PR description

Optional:

• [ X ] The code I wrote follows good style practices (see PEP 8 and PEP 20).
• [ X ] I have updated the docs in relation to my changes, if applicable
• [ X ] I have tested also GPU contexts

[giadarol]

Thank you! Would be nice to have a test added....

[carlidel]

Thanks! Sorry for the delay, I composed a simple test to check the self-consistency of the noise patterns. Of course the previous elens tests are all still compatible with this addition.