Restructure geometry structure

[pcarruscag]

Proposed Changes

Split geometry_structure.cpp/hpp in one file per class, strip spaces, replace tabs, move inlines to .hpp, etc.

This depends on #790, the only open PR affected by this is #773, @TobiKattmann I think it should be an easy merge since you only added new code.

Related Work

#583

PR Checklist

• I am submitting my contribution to the develop branch.
• My contribution generates no new compiler warnings (try with the '-Wall -Wextra -Wno-unused-parameter -Wno-empty-body' compiler flags).
• My contribution is commented and consistent with SU2 style.
• I have added a test case that demonstrates my contribution, if necessary.

[pcarruscag]

@talbring @rsanfer I noticed some of the new output files were missing the SU2 header, this PR also addresses that.

[talbring]

Thanks @pcarruscag ! Looks good.

[TobiKattmann]

Thanks for the heads up @pcarruscag . merging was no problem