Invert coordinates to plot lower triangle from upper triangle bricks

[Kelly-ST-HRI]

Adds invert_coords parameter. Backwards compatible with invert_coords = FALSE (default). Allows inverting axes to plot the transpose of brick object. Note the x_coords and y_coords match the x and y axes of plot created but the brick object contains data in the transpose matrix.

Follow up to PR #25 adding features discussed in Issue #18.

Enables:

• inverting the matrix to (optionally) allow plotting the larger chromosome on the x-axis from the same "brick" file

See here for the differences between #25 and #26
https://github.com/Kelly-ST-HRI/HiCBricks/compare/correct-coords...Kelly-ST-HRI:HiCBricks:invert-coords?expand=1

[Kelly-ST-HRI]

Defines invert_coords as follows:

invert_coords: Optional. Default FALSE.
A boolean (TRUE or FALSE) of length 1 specifying whether the coordinates in the
upper triangle of the data should be plotted transposed in the lower triangle.