Updating README

## Prerequisites

  - Required:

    - [NumPy](https://numpy.org/)

  - Optional:

    - [pytest](https://docs.pytest.org/en/6.2.x/)

## Configuring PyGriffin

## Installing

For now, the best method for installing PyGriffin is to add the top of the

PyGriffin repository to your `PYTHONPATH.

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

top directory) or by adding the locatiion of the PyGriffin repository to your

`PYTHONPATH` variable.

```bash

$ export PYTHONPATH=/path/to/pygriffin/repository

```

When installing with pip, PyGriffin's test dependencies can be included using:

```bash

$ pip install .[test]

```

## Testing

Move to the `tests` directory of PyGriffin and run:

With the testing prerequisites installed. Run the following command to test

```bash

pytest -v

```

## Detecting MPI

PyGriffin currently interrogates the MOOSE installation upon initialization to

determine if Griffin was built with MPI. To check if MPI parallelism is enabled

use the following command:

```python

>>> from pygriffin import PyGriffin

>>> pyg = PyGriffin('griffin.i')

>>> pyg.mpi_enabled

```

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