[Bug] echarts themes get treeshaken on production builds

[emoralesb05]

Bug Report

When running ng serve --prod the themes get treeshaken

particularly this line of code

covalent/src/platform/echarts/base/base.module.ts

Line 20 in e2fc638

registerDefaultThemes();

What is the expected behavior?

Themes should still be bundled when building the application.

Other information

We need to figure out a different way to register the themes if the current way is considered tree shakable.

Either have the users always add that line, add docs about it and we remove the call, or we make it work in all scenarios.