Last valid point as read_type

Demand for new functionality:

Example call:
df = c.read(['BA:CONC.1'], '05-Jan-2020 08:00:00', '05/01/20 11:30am', 60, read_type=tagreader.ReaderType.LastValid)

This should return data with a time step of 60 seconds. At each time step the data returned should be the last valid data point recorded before this.
Example: for the example call the first time step is 05-Jan-2020 08:00:00. The value given back should here be the last valid data point before this. This data point could be 1 second before or 1 month before, whatever is in the IMS.
The next time step is 05-Jan-2020 08:01:00. The value given back will be the same as for the first time step unless there is new valid time steps between 08:00:00 and 08:01:00. If there are several, the one closest to 05-Jan-2020 08:01:00 should be chosen.