Swift Package Manager Support

[jcavar]

Hello,
There are some changes in our project that will require FEM to be integrated using SPM. I managed to get this working in my fork but wanted to discuss solution before creating PR.

SPM has publicHeadersPath option where you can specify location of your headers. Unfortunately, it seems that it doesn't support wildcards and supports only one location.
As FEM's headers are distributed in different directories, this causes problems.

I think we have 2 options:

• Move everything under Source and remove directories (we can still have groups in Xcode)
• Move all headers under Source/Headers and keep existing directory structure

What are your thoughts?

[dimazen]

Hello, @jcavar! I would stick to moving everything into a single Source directory and leave just groups if that's the case.