Resolve "Port lalapps_heterodyne_pulsar to using only LALFrame for frame I/O"

Description

This attempts to resolve #436 (closed) by switching the lalapps_heterodyne_pulsar code to use LALFrStream functionality for accessing frame files, rather than directly using FrameL functions.

API Changes and Justification

Backwards Compatible Changes

• This change introduces no API changes
• This change adds new API calls

Backwards Incompatible Changes

• This change modifies an existing API
• This change removes an existing API

If any of the Backwards Incompatible check boxes are ticked please provide a justification why this change is necessary and why it needs to be done in a backwards incompatible way.

Review Status

cc @karl-wette @david-keitel