| freq | string | | Offset string. Detemining the frequency grid, the data shall be shifted to. |
| shift_method | string | `nearest_shift` | Method, used for data shifting. See a list of methods below. |
| drop_flags | list or Nonetype | None | Flags to be excluded from harmonization. See description of step 3 below. |
The function "harmonizes" the data-to-be-flagged, to match an equidistant
frequency grid by shifting the datapoints. This is achieved by shifting all data
points to timestamp values, that are multiples of `freq`.
In detail, the process includes:
1. All missing values in the data, identified by `np.nan`
get flagged and will be excluded from the shifting process.
NOTE, that implicitly this step includes a call to `missing` onto the
data-to-be-flagged.
2. Additionally, if a list is passed to `drop_flags`, all the values in data,
that are flagged with a flag, listed in `drop_list`, will be excluded from
harmonization - meaning, that they will not affect the further
interpolation/aggregation prozess.
3. Depending on the keyword passed to `shift_method`,
the data gets shifted, together with its flags,
to a timestamp that is a multiple of `freq`.
NOTE, that this step projects the data to an equidistant frequencie grid ranging from the initial to the last timestamp of the data passen and by this,
will very likely change the size of the dataseries to-be-flagged.
New sampling in the equidistant freq grid, covering no data in the
original dataseries, or only data that got excluded in step (1),
will be regarded as representing missing data (Thus get assigned `NaN` value).
The original data will be dropped (but can be regained by function
`deharmonize`).
`shift_method` keywords::
* `"fshift"`: every grid point gets assigned its ultimately preceeding flag/datapoint
if there is one available in the preceeding sampling interval. If not, BAD/np.nan - flag gets assigned.
* `"bshift"`: every grid point gets assigned its first succeeding flag/datapoint
if there is one available in the succeeding sampling interval. If not, BAD/np.nan - flag gets assigned.
* `"nearest_shift"`: every grid point gets assigned the closest flag/datapoint in its range. ( range = +/- `freq`/2 ).
