Tephogram methods should return axex for custom plotting by users... #6

At the moment, the Tephigram class return tephigram_python.plotter.Tephigram object which can not be customized by users. Therefore, request add functionalities to Tephigram class found in plotter.py module to return axes as an option. So that user can add their own axes to tephigram plot to customize to their liking.