Packaging restructured to support module imports

[sreekmtl]

Now module imports are supported in external python projects

[sreekmtl]

• Both Auth and STACAuth Token generators will now generate tokens by default on object creation and stores as object property
• Both auth objects are available globally , so no need of re-initialization or token generation seperately for each process
• New Auth APIs are updated in Readme
• This won't break any of CLI process
• Since auth is happening now outside context of process, ResourceFetcher class removed and instead the fetch_resource_data() is default way to download vector data
• Also updated docstring for few functions