parsers: create an NLM parser

[szymonlopaciuk]

Description

This is an implementation of a parser for the NLM format, it takes a very similar approach to the JATS parser which we already have, using LiteratureBuilder to build HEP records.

Related Issue

This is a step towards refreshing the IOP spider (#205)

Motivation and Context

IOP uses NLM format to publish their citation records. Currently the IOP spider uses web-scraping, however we will move to using OAI-PMH and this instead.

Checklist:

• [x] I have all the information that I need (if not, move to RFC and look for it).
• [x] I linked the related issue(s) in the corresponding commit logs.
• [x] I wrote good commit log messages.
• [x] My code follows the code style of this project.
• [x] I've added any new docs if API/utils methods were added.
• [x] I have updated the existing documentation accordingly.
• [x] I have added tests to cover my changes.
• [ ] All new and existing tests passed.