Psrqpy

- Add function to download and parse the galactic MSP table. See [80](https://github.com/mattpitkin/psrqpy/pull/80).
- Add functions to calculate derived gravitational wave parameters. See [79](https://github.com/mattpitkin/psrqpy/pull/79).

The major change for this release is that it no longer supports Python 2.7, and only supports Python versions
greater than 3.5.

- Add a function to download and parse the [globular cluster pulsar](http://www.naic.edu/~pfreire/GCpsr.html) table. See [#77](https://github.com/mattpitkin/psrqpy/pull/77).

- Allow `psrqpy` query to work using ATNF catalogue v1.64, which contains a position typo. See [74](https://github.com/mattpitkin/psrqpy/pull/74).
- More fixes to enable parsing of more references using NASA ADS and fixes changes that occurred with v1.64. See [70](https://github.com/mattpitkin/psrqpy/pull/70).

- 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).