Readd method ResourceProvider.json()

The method was replaced in 8837c03 and is now readded for backward
compatibility.
Use yaml.safe_load()

The load() method can execute any python function, which might be a
security issue in some usage scenario. Since we intend to use only
builtin types anyway, we are better off using the safe_load() function
for now, c.f.:

http://pyyaml.org/wiki/PyYAMLDocumentation#LoadingYAML
Remove typechecks in cpymad.model.save_model

The python idiom of choice in most cases is duck typing.

Typechecks should mostly be used to differentiate between alternating
implementations for different use cases. And even then inheritance is
often the preferred mechanism.

I believe that this function made an instance method of the Model class
or better be removed from the module altogether:

- it is trivial
- this module shouldn't need to know about how models are stored on disk
Add PyYAML as a install dependency in setup.py

Currently, requirements.txt is only used for build-time dependencies
that must be resolved before executing setup.py because they are
imported.
Fix comment implementation detail in test_resource
Use yaml.safe_dump
Export .yml files directly from modelConverter.py