Numerical formatting for axes

[trisyoungs]

Currently we are simply converting axis values to text using built-in Qt functions and not enforcing any decimal points or format etc. This is inflexible and can result in formatting artifacts (e.g. when displaying zero which can be interpreted as the smallest non-zero floating point value representable).

Need exponential / fixed form (switching between the two as necessary) and exponential / integer at the very least.