No explicit way to generate a DatetimeIndex with a infinite decimals frequency
[Link](https://stackoverflow.com/questions/48344540/high-and-not-round-frequency-datetimeindex) to the (unanswred) stackoverflow question which contains more details.

#### Problem description
The problem concerns the use of a step / frequency with infinite decimals. For instance to obtain a DatetimeIndex at a frequency of 300Hz one would try:

`pd.date_range(start=0, periods=300, freq='3333333ns') # start on UNIX epoch because I do not care`

with result
```
DatetimeIndex([          '1970-01-01 00:00:00',
               '1970-01-01 00:00:00.003333333',
               '1970-01-01 00:00:00.006666666',
               '1970-01-01 00:00:00.009999999',
               ...
               '1970-01-01 00:00:00.989999901',
               '1970-01-01 00:00:00.993333234',
               '1970-01-01 00:00:00.996666567'],
              dtype='datetime64[ns]', length=300, freq='3333333N')
```

#### Expected Output
As can be seen there is a deviation since at the end I would like to have a time o 00:00:00.996666666 (i.e. a step of 1/300=0.0033333333).

The [stackoverflow question](https://stackoverflow.com/questions/48344540/high-and-not-round-frequency-datetimeindex) proposes a workaround. But it results in an index that has no frequency (freq=None):

```
> date_range_fs(1, 300)
DatetimeIndex([          '1970-01-01 00:00:00',
               '1970-01-01 00:00:00.003333333',
               '1970-01-01 00:00:00.006666666',
                  '1970-01-01 00:00:00.010000',
               ...
                  '1970-01-01 00:00:00.990000',
               '1970-01-01 00:00:00.993333333',
               '1970-01-01 00:00:00.996666666'],
              dtype='datetime64[ns]', length=300, freq=None)
```

**My question is, would it be worth adding a Hz unit wich would automatically handle the problem of infinite decimals steps.** This would probably be achieved by keeping the frequency in the background and recomputing the steps only when needed.

Any interest on that ?