Specifying a sampling period via the sampling_period argument duplicated
multiplying the horizontal axes by a scaling factor.

This is because recent changes to postprocessor (see last paragraph of
commit) makes the columns of DataFrames show absolute time units (e.g. [0, 5,
10, 15...] if the images were taken every 5 minutes).  Previously,
columns showed time points (e.g. [0, 1, 2, 3...]).  All routines relied
on column labels to define the horizontal axis.  This issue would
have been addressed more timely if the commit in question had a more
informative description; 'add tiniterval property' tells me nothing and
does not inform me that it affects the column labels.

These recent changes mean that the sampling_period argument is no longer
necessary.  However, instead of deleting this argument, I've decided to
repurpose it for unit scaling, e.g. from minutes to hours.
Operationally, nothing has changed, but the meaning of the argument has
changed, and I've updated the docstrings accordingly.

This commit may affect horizontal axes of plots affected; users should
inspect the axes carefully, especially if the source data relies on
postprocessor.grouper.

This bug was likely caused by 17e13b59
on 2022-06-16 14:21.  It is unclear from the commit message, but I
suspect that this commit attempted to incorporate the image sampling
interval into defining the DataFrame columns produced by postprocessor.
This commit addresses issue #20.