```cpp
cvWriteMat_Base64(::cv::FileStorage & fs, ::cv::String const & name,
::cv::Mat const & mat)
```
becomes:
```cpp
CV_EXPORTS void cvWriteMat_Base64(::CvFileStorage* fs, const char* name,
const ::CvMat* mat);
CV_EXPORTS void
cvWriteMatND_Base64(::CvFileStorage* fs, const char* name, const
::CvMatND* mat);
```

Two other test are still needed.

1. Verify the Base64 data.
2. Read an old YML file for compatibility test.

1. Add Base64 support for reading and writing XML\YML file.

The two new functions for writing:

```cpp
void cvWriteRawData_Base64(cv::FileStorage & fs, const void* _data, int
len, const char* dt);
void cvWriteMat_Base64(cv::FileStorage & fs, cv::String const & name,
cv::Mat const & mat);
```

2. Change YML file header form `YAML:1.0` to `YAML 1.0`. (standard
format)

3. Add test for Base64 part.

[GSoC] FileStorage:
Add base64 support for reading and writting XML\YML file.
The two new functions:
```
void cvWriteRawData_Base64(cv::FileStorage & fs, const void* _data, int
len, const char* dt);
void cvWriteMat_Base64(cv::FileStorage & fs, cv::String const & name,
cv::Mat const & mat);
```

  * fix issue #6536
  * increase threshold in particular tests due to the difference of fmsub and fsub

  * fix issue 6521
  * use correct comparison

1) fix multichannel support
2) remove useless bad_value, read value from original Mat directly
3) add more tests
4) fix docs for cvCeil and checkRange

The original solution did not handle correctly when delta = 0,
resulting as nan errors. I also wrote a test case to test solving
equation x^3 = 0 after fixing.

calculates cube root of complex number to give more correct results.

fixes #5074

some mandatory string keys like paths must not be empty. Add the special
default value `<none>` so the CommandLineParser can enforce this and
generate an according error message for us.

requesting a previously undeclared key is most likely an programming
error. e.g. a typo "--unused vs --unsued".
So throw in those cases.

Add an according failure testcase.

(cherry picked from commit 7719da95)