Profile control - Added zMaxChars option that defines the maximum num…

[sebbelese]

When the z label is too large, it is partly written under the axis label or even outside of the canvas, see issue #748.

The proposed change considers adding a zMaxChars option that defines the maximum number of chars to be used for z graduation label before switching to scientific notation.

When the width of the largest label is larger than the zMaxChars limit, then, the whole axis switches to scientific notation with a single exponent deduced from the largest absolute value to be labelled.

The base numbers are then is displayed at a precision using maximum available characters defined by zMaxChars, including the decimal point and sign.

The proposed changes do not affect the default behaviour (i.e. no change if zMaxChars option is not provided), except a small increase of left margin that you may choose to ignore.

See a few examples for different orders of magnitude and zMaxChars=5:

[sebbelese]

Sorry, the latest commit should not have been included in the merge request, it was a change for my personal use, I removed it.