New save output

Saving diffraction intensities into different file formats
You can do all three of these is one block

```python
_HEADER = """
This file contains integrated powder x-ray diffraction
intensities.
Number of data points in the file : {n_pts}
 ########################################################################################
"""
```

and then here all you need is

```python
_encoding_writer(f, _HEADER.format(n_pts=len(tth)))
```