wrap file open in a with statement

[mmillns2]

This PR addresses issue #54: Inappropriate file opening

• This PR wraps a file open in a with statement.

Implementation

• Previously we had something like:

file = open(file_path, 'rb')
...
file.close()

• The issue with this is that if for some reason the code breaks between the open and close, the file remains open. This is unsafe.

• To make this safe, a with statement can be used to ensure correct file closing:

with open(file_path, 'rb') as file:
    ...

[jwaiton]

This PR resolves an unsafe implementation of file opening, good work!