make an AstroDBHeader class and methods

From @dr-rodriguez :
```python
class AstroDBHeader(Header):
def __init__(self, header=None):
# prepare empty header if none is provided
def add_missing_keywords(self, format, keywords):
# add missing keywords
# etc etc
```
what that allows you to do is essentially add extra methods to your original header so you can do things like

```Python
header = AstroDBHeader()
header.add_missing_keywords()
header.check_header()
```
and everything else defined in `fits.py`

Start by reading the existing Header implementation and related methods in fits.py. Define the requested AstroDBHeader subclass with optional-header initialization and the described add_missing_keywords behavior while retaining the existing Header API. Done means the example calls work and the inherited fits.py functionality remains available.

Written by the indexing model from the issue text.