BUG: disallow 'w' mode in pd.read_hdf (#13623)
- [x] closes #13623 
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry
  
  Passing mode='w' to read_hdf raises ValueError.

needs tests
 ## [Current coverage](https://codecov.io/gh/pydata/pandas/pull/13858?src=pr) is 85.25% (diff: 100%)

> Merging [#13858](https://codecov.io/gh/pydata/pandas/pull/13858?src=pr) into [master](https://codecov.io/gh/pydata/pandas/branch/master?src=pr) will increase coverage by **<.01%**

``` diff
@@             master     #13858   diff @@
==========================================
  Files           140        140          
  Lines         50455      50457     +2   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
==========================================
+ Hits          43014      43017     +3   
+ Misses         7441       7440     -1   
  Partials          0          0          
```

> Powered by [Codecov](https://codecov.io?src=pr). Last update [59f2557...b03d90b](https://codecov.io/gh/pydata/pandas/compare/59f25575f42e19f066dc3cd2245983071e2d7813...b03d90b73c63deb13b0e87a9034420e118abbb38?src=pr)
 The existing test cases check for r, r+, a and w modes. I have added a new test case to check read_hdf for default mode (i.e without mode argument given).
 @iamsimha minor changes. ping when green.
 @jreback I have made the changes.
 thanks @iamsimha 
