split Fortran2008.py into one file per class

[rupertford]

The Fortran2008.py file is getting large and it would be better to have a Fortran2008 directory containing a separate file for each class with an init that imports all of the files. There may be an issue with the class generation and class hierarchy generation.

[rupertford]

I tested this out in https://github.com/stfc/fparser/tree/split_f2008_file and have got it to work. I'll wait until #408 is on master before progressing this.

[rupertford]

I'll create a Fortran2008 directory containing an init file and move the Fortran2008.py file into this directory first before splitting out the classes as I also need to move the class generation code into the init file and update the logic that calls this.

[rupertford]

Created branch 411_move_fortran2008_file

[rupertford]

Now that #417 is on master I can split the Fortran2008.py file into one file per class.

[rupertford]

Created branch 411_split_file