to_json integration

[khider]

Is your feature request related to a problem? Please describe.
Ability to save to json files

Describe the solution you'd like
Right now, the json implementation is not at the API level. Would be nice to allow for it

[khider]

Enabled for the Series object in fd24e07. Needs to be done for other objects after #298 is complete

[khider]

Fixing this issue will also fix #408 and #298

Right now, the json utils are low-level utility functions that need to be accessed through the utils API. This presents significant issues: (1) need to import the package within the json utils module to avoid circular importing (making import of Pyleoclim slow), (2) can't properly check for valid keys in the objects. Putting at the API level would solve this problems.