Nuclyr

`nndc`
* first implementation of new submodule
* first feature: obtain level energies and their uncertainties via `nndc.levelscheme.energies()`
* non-formated level scheme information can be accessed via the pandas dataframe nndc.levelscheme.df

`mass`
* fixed bug with neutron mass excess
* new method `mass.qValue(Z1,A1,Z2,A2,Z3,A3,Z4,A4)

* New submodule `mass`
* New feature mass.massExcess(Z,A) to obtain nuclear mass excesses
* New feature mass.abundance(Z,A) to obtain natural isotopic abundance

Basic functionality of the config and exfor submodules to scrap data from the EXFOR database.