Ampal

Bug fix for slicing `Polypeptides`.

* Adds rotamer classification with the classify_angle_as_rotamer in the analyse_protein module
* Fixes a bug with parsing structure files

* **Adds `get_ss_regions` to `ampal.dssp`.** This function can be used to
extract all regions of a protein in a particular secondary structure.
* **Fixes bug with DSSP `ss_region` tagging.** End residues used to be missed.

**Adds an interface for NACCESS.** Functions for using NACCESS to calculate solvent accessibility.

Fixes small bug in the order that `isinstance` is called when creating a `Monomer`.

Initial release of AMPAL in [PyPi](https://pypi.org/project/AMPAL/). Contains most functionality from the AMPAL module of [ISAMBARD](https://github.com/woolfson-group/isambard/tree/master/isambard/ampal), but has been split out of the main project to allow people to use and develop AMPAL independently from ISAMBARD.