PrairieReader: avoid NPE from missing attributes

If linesPerFrame or pixelsPerLine is missing, we fall back to using the
first TIFF file's Y or X size, respectively.
PrairieReader: Add PrairieMetadata#getSequences

This method returns the list of Sequences ordered by key. It will be
useful to handle cases where the cycle numbering does not increment one
at a time (e.g., for metadata with cycle=2,5,8,11,... or even for
non-linear increment patterns).
PrairieReader: fix bug in logging statement
PrairieReader: handle any sequence numbering

PrairieView is capable of producing data where the cycle numbering does
not increment one at a time (e.g., for metadata with
cycle=2,5,8,11,...). We have not yet observed any datasets with
variable, non-linear increment patterns (e.g., cycle=2,4,5,11,12,...),
but this new approach should handle that too.

The new appraoch works by explicitly creating and sorting a list of
existing sequences, then using that list as the basis for sizeT & sizeP,
rather than assuming that Sequence#getCycleCount() will be sensible.
PrairieReader: tweak javadoc
PrairieReader: fix handling of inverted Z/T

In the case where a Sequence is flagged as a TSeries, the Frames of that
Sequence must be treated as time points rather than focal planes. This
logic was previously not fully propagated through the code.