feat(expansion-panel): introducing expansion-panel-group to only give margin to expansion panels when grouped

[emoralesb05]

Description

this will give spec margin to grouped expansion panels following the spec, but its optional so we can use stand alone expansion panels without margin too for other use cases.

What's included?

• new component TdExpansionPanelGroupComponent so we can group expansion panels and add margin following material spec

Usage

<td-expansion-panel-group>
  <td-expansion-panel label="Label goes here">
    <div class="md-padding">
      <h3 class="md-subhead">md-padding class</h3>
    </div>
  </td-expansion-panel>
  <td-expansion-panel label="Another label goes here">
    <div class="md-padding">
      <h3 class="md-subhead">md-padding class</h3>
    </div>
  </td-expansion-panel>
  <td-expansion-panel label="Yet another label goes here">
    <div class="md-padding">
      <h3 class="md-subhead">md-padding class</h3>
    </div>
  </td-expansion-panel>
</td-expansion-panel-group>

Test Steps

• Go to http://localhost:4200/#/components/expansion-panel
• See 1st demo without group and 2nd demo with group

General Tests for Every PR

• ng serve --aot still works.
• npm run lint passes.
• npm test passes and code coverage is not lower.
• npm run build still works.