Psrqpy

- Allow the `Pulsar` objects to access parameter references. See [71](https://github.com/mattpitkin/psrqpy/pull/71).

- A minor fix to get ADS references to work. See [66](https://github.com/mattpitkin/psrqpy/pull/66).

Changes for this release:

- The way references are stored in the ATNF pulsar catalogue has changed, so the `get_references` function has been changed to work with this new format. This breaks that function for older cached catalogue, but a warning is provided. See [64](https://github.com/mattpitkin/psrqpy/pull/64).
- The astropy Galactocentric coordinates have been set to use default values from the pre-v4.0 release. See [65](https://github.com/mattpitkin/psrqpy/pull/65).

Changes for this release:

- Fix an issue with Tables in Astropy v4.0 (see [61](https://github.com/mattpitkin/psrqpy/pull/61)).

Changes for this release:

- Adds position errors on the right ascension and declination when converted into degrees (see [60](https://github.com/mattpitkin/psrqpy/pull/60)).

Changes for this release:

- The version 1.61 of the ATNF Pulsar Catalogue containing the declinations with the unicode "−" character in place of an ascii minus sign "-" has been fixed upstream, so the code fix to account for this has been removed (it was also causing issues with the Python 2 version of the code).