Adding simple test that isotopes are extracted from the XML file correctly

<ISOXML>

  <Multigroup_Cross_Section_Libraries Name="ISOTXS-neutron" NGroup="33">

    <Multigroup_Cross_Section_Library ID="1" Ver="1.0" Generator="INL" TimeCreated="Wed Jul 20 21:02:47 2022" Description="ISOTXS">

      <Tabulation>Tfuel</Tabulation>

      <Tfuel>300</Tfuel>

      <ReferenceGridIndex>1</ReferenceGridIndex>

      <AllReactions>Total Transport Removal Absorption NGamma N2N NAlpha NProton NDeuteron NTriton Fission nuFission kappaFission EnergyDeposition FissionSpectrum NeutronVelocity Scattering</AllReactions>

      <TablewiseReactions/>

      <LibrarywiseReactions/>

      <Table gridIndex="1">

        <Isotope Name="pseudo_REG_A" L="1">

        </Isotope>

        <Isotope Name="pseudo_REG_B" L="1">

        </Isotope>

        <Isotope Name="pseudo_REG_C" Class="Structure" L="1">

        </Isotope>

      </Table>

    </Multigroup_Cross_Section_Library>

  </Multigroup_Cross_Section_Libraries>

</ISOXML>

import pytest

from pygriffin.isoxml_reader import parse_isoxml

def test_isoxml_materials():

    exp_materials = ['pseudo_REG_A', 'pseudo_REG_B', 'pseudo_REG_C']

    materials = parse_isoxml('./test_files/isoxml_sample.xml')

    assert exp_materials == materials

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