WIP: Code style conventions

Fortran style guide

There are resources available online on best practices when writing modern Fortran code:

Grasp2k-specific conventions:

New code should be written in the Fortran 90 free-form style. Code should may use Fortran 2003 features (i.e. it is expected that compilers will support Fortran 2003).
Variable names, Fortran statements etc. should be written in lowercase. Preprocessor directives should be uppercase.
Underscores should be used to make long names readable (e.g. long_variable_name)
Lines should wrapped around the 80-character mark. 92 characters should be considered a hard limit, although exceptions are OK if it improves readability.
There should be no extra whitespace at the ends of lines, nor should there be extra empty lines at the ends of files.
Indentation should be done with four spaces.
Fortran 77 code must use the .f extension, modern code should use the .f90 extension.

Proposal: to make it more clear that the f77 code is legacy, we could switch to .f for modern free-form code and use .f77 for the old code.
In order to have a common namespace, Grasp2k-specific modules should be prefixed with g2k_ (e.g. g2k_quad).
Kinds should be used for floating point variables. For 64-bit floats, the real64 and dp parameters should be defined once and then used (i.e. real(real64) :: ... for declarations, 2.341_dp for floating point literals). They can (should) be defined via the iso_fortran_env intrinsic as follows:
```
use, intrinsic :: iso_fortran_env, only: real64
integer, parameter :: dp = real64
```