* created new decoder and encoder for PFM

pfm file format stores binary RGB or grayscale float images.

* added test for pfm codec

* replaced large with short licence header

* little/big-endian-check is now compile time

* fixed width/height confusion, improved big/little endian recognition, fixed scaling issue and Improved signature check

* adapted tests to handle float images well

* removed data-dependency: lena.pfm

the lena image is now loaded is pam and converted to pfm.

* fixed bug in endianess detection macro

* Added endianess detection for android and win

* removed fancy endianess detection

endianess detection will be implemented in cmake scripts soonish.

* fixed minor warnings

* fixed stupid elif defined bug

* silenced some implicit cast warnings

* replaced std::to_string with std::stringstream solution

std::to_string variant did not build on android.

* replaced new endianess macros with existing ones

* improved readability

	* Keep image aspect ratio in resize called in grad_system and
weak_order
	* Bug correction in loop inside Decolor::gradvector

* core(test): merge hang test

* core(merge/split): fix intrin optimization

fixed call open(int id) failed

All vx_call() must be wrapped into own simd128/simd256/simd512 namespace

```
namespace CV__SIMD_NAMESPACE {
... vx_call declaration is here ...
}
```

- use '3.4.x' cache name for current maintenance series (there are no serious changes between releases)
- message is shown only once during creation of new cache directory
- use OPENCV_CACHE_SHOW_CLEANUP_MESSAGE=0 to hide this warning

imgproc: update cornerSubPix documentation

* I have updated the requirements of reporting issues inside ISSUE_TEMPLATE.md