Psrqpy

- Set position epoch to period epoch if the latter is not given. See [96](https://github.com/mattpitkin/psrqpy/pull/96).
- Allow discovery data tp be derived from the catalogue. See [97](https://github.com/mattpitkin/psrqpy/pull/97).

- Allow specific versions of the ATNF pulsar catalogue to be downloaded. See [95](https://github.com/mattpitkin/psrqpy/pull/95).

- Extend `__getitem__` for `QueryATNF` class so that individual pulsar can be extracted using keys from a class instance. See [92](https://github.com/mattpitkin/psrqpy/pull/92).

- Propagate errors on ecliptical coordinates to equatorial. See [90](https://github.com/mattpitkin/psrqpy/pull/90).
- Update build system. See [91](https://github.com/mattpitkin/psrqpy/pull/91).

- Move scraping of Galactic MSP catalogue into PSRQPy. See [88](https://github.com/mattpitkin/psrqpy/pull/88).

- Fix parsing of the galactic MSP table after changes to the upstream format. See [85](https://github.com/mattpitkin/psrqpy/pull/85).
- Remove explicit use of certain NumPy built-in types following their [deprecation](https://numpy.org/devdocs/release/1.20.0-notes.html#deprecations). See [83](https://github.com/mattpitkin/psrqpy/pull/83).