Update SPECFEM++ with Google Style guide

[Rohit-Kakodkar]

Task description

This issue tracks the updates needed to SPECFEM++ to be consistent with Google Style guide.

Here are a list of styling guidelines we'd follow after this refactor

• Headers
  • Include what you use :
    • If a source or header file refers to a symbol defined elsewhere, the file should directly include a header file which properly intends to provide a declaration or definition of that symbol.
    • It should not include header files for any other reason.
    • Do not rely on transitive inclusions.

Please update this issue as you see fit

Acceptance criteria

• The tests for the new function bar() pass