Add DE436 ephemeris to those allowed by the known pulsar code.

Description

This MR allows the known pulsar code to use a Sun/Earth ephemeris file created using the DE436 ephemeris. However, the MR does not add new ephemeris files for the DE436 ephemeris to the repository.

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 @david-keitel @karl-wette