Rename CNodes as LNodes, add typing, process AST separately from creating it.

[chrisrichardson]

This PR adresses removes CNodes, and replaces with a much slimmed-down "LNodes" which contains the basics:

• Variables and Arrays of Scalar (same as tensor), Real (same as geometry) and Int
• Basic math operations +-*/
• Math functions, including conditional statements
• ForRange loops

but removes almost everything else.

• All Symbols must now have a DataType

Additionally, the Abstract Syntax Tree (AST) which is generated is now processed separately in c_implementation.py, which will allow us to post-process into other languages/libraries in the future.

This is quite a large PR, unfortunately, but I think it will move things in the right direction.

[coveralls]

coverage: 76.575% (-2.0%) from 78.566% when pulling 5198ba64ff90652ed0d5f4dd4bd895354c70a0ef on chris/lnodes into 4572bec1d9d67bbaa711ec4d09e80935e0d84f99 on main.

[chrisrichardson]

I just added some formatting for complex numbes and variable precision. Standard C needs the 1.0+1.0*I rather than 1.0+1.0j although modern compilers seem to accept either.