Add Elstob font family

This is a variable font in roman and italic, covering all European languages using Latin script but aimed particularly at medieval scholars. It is based on one of the "Fell" typefaces used by the Oxford University Press in the late 17th and early 18th centuries, specifically the Double Pica cut by the Dutch designer Peter De Walpergen.

The aim of this project has been to create a practical, legible typeface that will display well on screen or in print, with a curated selection of medieval characters (not the vast MUFI set, for which users can go to my Junicode font, but generally useful characters with Unicode code-points). Also OpenType features providing conveniences (e.g. automated transliterations) for medievalists.

The font has three axes: Weight, Optical Size, and Grade.

I have read the guidance for contributors and believe I am in compliance. At this point the font passes all fontbakery tests except for two: the dropout control test (this is because I hinted with VTT, which obfuscates the prep code by doing 100+ pushes at the top of the program--but dropout control is properly set), and the requirement that the default Optical Size be zero. I am willing to adjust the Optical Size scale, but thought it best first to have a discussion of why fontbakery's requirement is so at odds with ([the published OpenType spec](https://docs.microsoft.com/en-us/typography/opentype/spec/dvaraxistag_opsz)).

I have been involved in type design for a long time, but I am amateur and self-taught, and therefore happy to address any design or QA issues that come up. The repository is [here](https://github.com/psb1558/Elstob-font), and a specimen page with the obligatory sliders is [here](https://psb1558.github.io/Elstob-font/).