include docstring in pyprika package __init__
get started with sphinx documentation generation
lots of documentation work
add section headings, exceptions, and ivars
Merge branch 'feature/docs' into develop
remove unconstrained attributes

these may show up in later versions but are useless as unconstrained fields
unless they will always be unconstrained
add `make test` rule
use deep copy value
add unit test for dump; fix an exposed error
change spec of pyprika.load

no other module supports loading file paths so it seems best not to support
it
load/dump have drop in callables to convert to dict
add option to Recipe.to_dict to pre-serialize
documentation for dump() and dumps()
load recipe into file
log a file which fails to load

add options to silence logs
lazy-load recipes
fix some issues with pprint_recipe

- handle tuple-style 'servings' field properly
- don't print 'Source: ' twice
make quantity._to_number handle mixed numbers
add __mul__ for Recipe and test cases
make QuantityDescriptor force acceptance of name