Retrieve new code from my old refactor class tree branch

I started to integrate some of the code I added in my abandoned refac class tree branch (began with #37 (closed)).
It's a big diff since I moved functions from App to otbObject, and reorder some special methods def so they are grouped.
No changes in the exec logic, all test passed with release 8.1.

Changelog :

• Find OTB lib during module init or die
• Raise SystemExit if no applications were found (print message to set OTB_APPLICATION_PATH)
• Small enhancements (style and variable names)
• Smaller apps.py (avoid duplicated OTBTF app class def)
• Break backward compat to conform to camel case : logicalOperation => LogicalOperation | otbObject => OTBObject
• Operation and Slicer are OTBObject subclasses, see attached class tree
• Keep function def order logical : init, properties, public methods, private methods, special (dunder) methods
• Move more code from App to OTBObject. App should only be used for UI specific functions. All the app init logic is now in OTBObject
• Move App class def to apps.py
• Add OTBObject.flush to control the WriteOutput / ExecuteAndWriteOutput logic
• Remove OTBObject's preserve_dtype init arg, move it to write()
• Make OTBObject.propagate_dtype() public
• Add OTBObject functions and properties to ease app settings :
  • store all parameters names in OTBObject.parameters_keys
  • store output param keys in OTBObject.out_param_keys
  • store output type (raster, vector, file) of each output param in OTBObject.out_param_types
  • app.key_input, OTBObject.key_input_image, OTBObject.key_output_image to easily find parameter keys (replace old app.output_param)
  • thanks to OTBObject.key_input, it is now possible to pass most input parameters without key, as we do for in or il (for example io.in or ingeom should work)
• Add attributes and functions to ease data exchange with numpy / rasterio :
  • store exported numpy arrays in OTBObject.exports_dict, thus we can avoid exporting arrays multiple times, ExportImage() is only called once for each key
  • add app.transform property (see #54 (closed))
• Move boilerplate code out of OTBObject because it's way too big (utils and static methods : is_key_list, is_key_image_list, get_out_param_types, parse_pixel_type, etc.). It also allow us to re-use these, no reason to keep them private.

Before :

After:
Fix #37 (closed)