Adding xml parsing for particles

[JamesEdgeley]

Description

Desirable to have flexible particle spec and other particle parameters for coupling to reactions

Fixes # (issue)
All particle input to solver to be specified in solver config e.g.:

<NEKTAR>
    <PARTICLES>
        <INFO>
            <I PROPERTY="PARTTYPE" VALUE="ReactionSystem"/>
        </INFO>
        <PARAMETERS>
            <P> particle_output_freq = 1</P>
        </PARAMETERS>
        <SPECIES>
            <S ID="0" NAME="Ar">
                <P PROPERTY="Mass" VALUE="1.0"/>
                <P PROPERTY="Charge" VALUE="1.0"/>
                <P PROPERTY="Number" VALUE="1000"/>
        </SPECIES>
    </PARTICLES>
</NEKTAR>

Particle systems to piggy-back Nektar++ factory creation method to allow initialisation of particle_group to be delayed until after list of species and reactions is known.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change)
• New feature (non-breaking change which adds functionality)
• API change (breaks downstream workflows and requires major semver bump)
• Requires documentation updates

Testing

Please describe the tests that you ran to verify your changes and provide instructions for reproducibility. Please also list any relevant details for your test configuration.

• Test Foo in /test/path/to/file_for_test_Foo.cpp
• Description of test Foo
• Test Bar in /test/path/to/file_for_test_Bar.cpp
• Description of test Bar

Test Configuration:

• OS:
• SYCL implementation:
• MPI details:
• Hardware:

Checklist:

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any new dependencies are automatically built for users via cmake
• I have used understandable variable names
• I have run clang-format against my *.hpp and *.cpp changes
• I have run cmake-format against my changes to CMakeLists.txt
• I have run black against changes to *.py
• I have made corresponding changes to the documentation
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings