Changes required for removing ParticlesBase

[szymonlopaciuk]

Description

1. Remove the particles_class argument from some methods that no longer require it, following the proposed changes to xtrack and xpart (Move particles to xtrack, remove ParticlesPurelyLongitudinal xtrack#449).
2. Put materials on a context different from context_default. This is needed, as otherwise whenever xcoll is imported, the materials are allocated on the default context. This is not in itself wrong, but causes a problem for the test run of xtrack, where emptiness of the default context is checked (as a means of detecting memory leaks -- this was a problem in the past). Rather than disabling the check, we can allocate the materials on a different context, as a means of specifying that these are valid allocations. Maybe a better solution can be found, but for now this seems the best.

Closes # .

Checklist

Mandatory:

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

Optional:

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