General pxweb api

Now the pxweb package can handle any pxweb api.

News:

• Jordbruksverket and Örebro pxweb api added
• Aliases added
• Possibility to create, add and test new apis added using the pxweb_api class
• Api descriptions added
• interactive_pxweb() without any arguments can be used to navigate between apis.
• Package check for new apis on startup

Bugs:

• Extensive test suite added
• bug fixes