Improved colorbar limits selection

Solves Issue #22

This PR allows for numbers with any number of decimal places to be entered as limits for the colorbar. It also allows for the numbers to be entered in scientific notation. 

This is achieved by changing the qSpinbox that had previously been used to a qLineEdit. The string is then converted into a number using the inbuilt 
```
float(sting) 
```

As such, the expected scientific notation is 2.7e-5, however I have also accounted for adding spaces at any point.