Correct doc strings for Tempo par file reader and add additional parameters

Detailed Description

The doc strings for the parameters BETA and LAMBDA that can be read in from a Tempo-style parameter file are currently wrong - the doc strings states that these are Galactic latitude and longitude, when really they are just aliases for the ELAT and ELONG values, which are the ecliptic coordinates (see Table 2 of the Tempo2 manual). This MR corrects these doc strings.

In addition, it also adds in the ability to read in a couple of other potentially useful parameters from a par file (TIMEEPH and T2CMETHOD) and add checks to the test code to read in some additional parameters.

API Changes

Please tick one of the following options:

• These changes do not modify the API.
• These changes do modify the API, and are backwards compatible.
• These changes do modify the API, and are backwards incompatible.

For examples of changes that do not modify the API and/or are considered backwards (in)compatible, please see the contributing guide.

Justification for Backwards Incompatible Changes

Review Status

cc @david-keitel @karl-wette