documentation changes

[FDauphin]

Apologies if I'm not using standard SWE vocabulary or mixing up words where necessary...

Major changes:

• all help files are removed
• all modules have combined content from help files, wfc3tools readthedocs, and WFC3 DHB as documentation
• documentation cross references WFC3 DHB where applicable
• Pyraf support in documentation removed
• stsci.tools.teal support removed
• Python 2 dependencies (e.g. from future import division) removed
• readthedocs and module documentation are consistent with each other
• README on main page edited
• wfc3tools/test moved out of main package and into main repo
• removed wfc3tools/pars (can easily put it back if necessary)
• added scripting functionality to most py files (i.e. if name = 'main':)

Not sure of:

• calwf3 doc file is too advanced; need someone with more experience to make sure it is consistent with actual calibration routine
• some functionality questions in py files that need to be answered (e.g. do we need some parameters if they aren't being called in the Python wrapper?)

We need to add on main README the necessary details that wfc3tools is a Python wrapper and you need(?) hstcal already installed.