ImageXpress: support time dimension

[imagejan]

This adds an optional TimePoint_(?P<t>\d+) to the ImageXpress patterns, respects t when computing the tile position, and adds a t dimension to the well acquisition axes if applicable.

Remaining task:

• add test dataset with multiple time points, and assert that well image shape corresponds to the reported axes.

[imagejan]

@fstur the changes in 241f723 (#121) should address your use cases, I suppose. They relax the regex patterns to allow for optional TimePoint_* subfolders (that are always created in newer versions of the ImageXpress acquisition software).

/cc @jluethi

[fstur]

@imagejan thanks! I will have a look.