DOC: Simplify study dictionary creation

[Leengit]

Instead of assigning each individual dictionary key-value pair in its own Python expression, more concisely and clearly assign them all in one combined Python expression, using the standard dict(...) constructor. (This is related to Issue #94 but does not close it).

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[Leengit]

The second commit shortens tile_overlap_width to overlap_width and tile_overlap_height to overlap_height.

[Leengit]

The recent force push of the second commit additionally records these overlap values in the study and slide dictionaries so that the user need not try to derive the overlap values from the resulting tile positions. Slides can have and record different overlap values from each other. The study-wide value is either set directly by the user, or is taken from the first use of TilesByGridAndMask in the study. Creation of a TilesByGridAndMaskobject without specifying overlaps now defaults to the study-wide values, if available, else zero. Previously the default was simply zero. This closes Issue #93.

[Leengit]

Force push just now, rebases to current master.

[Leengit]

The third commit closes Issue #104.