Fix image number calculation for grid scans

[DominicOram]

If I have a gonio with axes like:

"starts": [0.0],
"ends": [2.0],
"increments": [0.2],

then calculate_grid_scan_range calculates that there will be 10 images but actually I think there should be 11:

0.0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0

Subsequently when we pass this to scanspec, because we're assuming 10 images, it then gives us:

0.0, 0.222, 0.444, 0.666, 0.888 ... 2.0

Which disagrees with the increment number we've passed

[DominicOram]

There are also issues with floating point calculations. e.g.

    "starts": [420.9968],
    "ends": [1040.9968],
    "increments": [20],

gives a image number of 30

[noemifrisina]

In part fixed by #59 but it still needs a couple of fixes when using increments instead of number of images for a grid scan.