unify loading of glyphs source into GSFont object

[anthrotype]

we currently have two alternative ways:

1. instantiating a new GSFont object and passing a path to a .glyphs file

from glyphsLib import GSFont
font = GSFont("MyFont.glyphs")

2. using the json-like glyphsLib.load (for a file or path,loads for a string) function

import glyphsLib

with open("MyFont.glyphs") as fp:
    font = glyphsLib.load(fp)

The two methods supposedly produce the same kind of object, however they seem to be calling different methods of glyphsLib.parser.Parser.

this is in GSFont.__init__:
https://github.com/googlei18n/glyphsLib/blob/0a86327a93b3f324f200e35a23f639db4ae58280/Lib/glyphsLib/classes.py#L3010-L3013

this is loads function:
https://github.com/googlei18n/glyphsLib/blob/0a86327a93b3f324f200e35a23f639db4ae58280/Lib/glyphsLib/parser.py#L255-L263

[schriftgestalt]

I don’t like the loads function ;)

And how it is done in Defcon and ufoLib? No loads functions there as far as I see.

[belluzj]

I don't know much about the Python conventions, so I don't know how "pythonic" and "expected" it is to provide the loads function, but personally I find myself using the GSFont constructor all the time :)

[anthrotype]

let's deprecate them, and consistently use only one in our codebase. currently load_to_ufos is using the load function.

[khaledhosny]

I find it a good idea not to force filesystem io use unnecessarily; if loading from a string is possible then it should be kept supported. Duplicated code paths can be fixed by making one of the methods call the other.

…

[anthrotype]

GSFont can have a fromstring class method constructor to load from a string

[schriftgestalt]

Or add a keyword argument to the constructor.

font = GSFont(fromString=string)