Adding frontend for JPEG encoder.

----

This commit includes some changes in the existing files and adding some new files as follows:

**Files which are edited**
  ```
  modified:   litejpeg/core/common.py
```
              
"common.py" contains the various datasets required for initialising the inputs and outputs in other modules. Additional documentation is added to this to make it clear about  which dataset is used in which block and how many bits are assigned to each.
```
  modified:   litejpeg/core/csc.py
  modified:   litejpeg/core/quantization.py
  modified:   litejpeg/core/zigzag.py
  modified:   test/Makefile
```

Additional documentation is been added to make the code more readable.
```  
  modified:   test/common.py
```

Contains the input_data for testing and functions for conversion of data and output in the required format.
Input_data is been added for the testing of the RGB2YCbCr, DCT, Quantizer and Zigzag module.
Added functions such as pack_dct(), unpack_dct() to get the data or the output of the implemented module in the required format.


**Newly added files** 
       
``` 
litejpeg/core/new_dct6.py
```

"new_dct6.py" contains the DCT module required for the conversion of the YCbCr in order to suppress the high frequency component present in the matrix. This is been added as the older version that is "dct.py" is not been working over the "new_dct_tb2.py" giving compilation errors. 
Even after the addition of this module we can remove the dct.py as per requirement.

``` 
test/model/enc_frame.py
```
"enc_frame.py" contain the software implementation of each module for the purpose of testing. The 
 ouptut of each implemented module is compared with the software version to check the accuracy of 
 the implemented module. 

```
test/new_dct_tb2.py
test/new_quantize_tb.py
test/new_zigzag_tb.py
```

These three test benches are added to check the accuracy of the DCT, Quantizer and ZigZag module respectively. The test bench runs the same set of inputs to the software and implemented module and compare the output of both.



After adding the above changes, the resultant repository will have **working model of RGB2YCbCr, DCT , Zigzag and Quantization module along with the Test Benches** for each of them.