Metadata indecies of UngriddedData are offset relative to the metadata dictionary in some cases

[thorbjoernl]

Describe the bug
Please provide a clear and concise description of what the bug is.

• Pyaerocom version: '0.22.0'
• Computing platform: NA
• Configuration file (if applicable): NA
• Error message (if applicable): NA

To Reproduce
The following script reproduces the error:

import pyaerocom as pya
import pandas as pd
import numpy as np

if __name__ == "__main__":
    VAR = "vmro3max"
    obs_reader = pya.io.ReadUngridded("EBASMC")
    data = obs_reader.read(vars_to_retrieve=VAR)
    data = data.filter_by_meta(ts_type = "hourly", station_id = "NO0001R")
    df = pd.DataFrame()  
    df["time"] = data._data[:, data._TIMEINDEX].astype("datetime64[s]")
    print("UNIQUE METADATAINDEXES:")
    print(np.unique(data._data[:, data._METADATAKEYINDEX]))
    print("METADATA KEYS:")
    print(data.metadata.keys())
    df["station_id"] = [data.metadata[x]["station_id"] for x in data._data[:, data._METADATAKEYINDEX]]

Expected behavior
Metadata key lookup should work without error.

Screenshots

UNIQUE METADATAINDEXES:
[4638. 4639. 4640. 4641. 4642. 4643. 4644. 4645. 4646. 4647. 4648. 4649.
 4650. 4651. 4652. 4653. 4654. 4655. 4656. 4657. 4658. 4659. 4660. 4661.
 4662. 4663. 4664.]
METADATA KEYS:
dict_keys([0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0, 13.0, 14.0, 15.0, 16.0, 17.0, 18.0, 19.0, 20.0, 21.0, 22.0, 23.0, 24.0, 25.0, 26.0])
Traceback (most recent call last):
  File "/home/thlun8736/Documents/work/aerotools-scripts/bug.py", line 16, in <module>
    df["station_id"] = [data.metadata[x]["station_id"] for x in data._data[:, data._METADATAKEYINDEX]]
  File "/home/thlun8736/Documents/work/aerotools-scripts/bug.py", line 16, in <listcomp>
    df["station_id"] = [data.metadata[x]["station_id"] for x in data._data[:, data._METADATAKEYINDEX]]
KeyError: 4638.0

Additional context

• Unique metadata key indexes has the same number of sequential entries as the dictionary, so it seems likely that some erroneous offset is being applied.
• UngriddedData.merge may be where this issue occurs as it is the only place which seems to apply an offset.