Adding additional information on setting and checking the griffin executable

These will be preferentially used by PyGriffin in the order they are provided by

the user and the user's compute environment.

### The `.pygriffin.rc` file

The `.pygriffin.rc` file uses a configuration format commmonly adopted

by Python modules for customization and is supported by a a parser in

the Python standard library. Specifics on this format can be found

[here](https://docs.fileformat.com/system/ini/).

PyGriffin expects to find an `exec` section containing the location of

the Griffin executable and (optionally) the ISOXML executable. For

example, a `.pygriffin.rc` file on a system with Grifin installed at `/home/griffin_project/`

might look like:

```

[exec]

griffin = /home/griffin_project/griffin-opt

isoxml = /home/griffin_project/isoxml/isoxml-opt

```

To check that PyGriffin has discovered the correct executable, use the

following commands in a Python interpreter:

```python

import pygriffin

pyg_config = pygriffin.PyGriffinConfig()

print(pyg_config.griffin_exec)

```

## Installing

PyGriffin can be installed by using pip (`pip install .` from the repository's