Add feature that allows mathematical use of brackets when defining channels

[l-a-smith]

Currently, basic math operations can be used when defining channels, ie. addition,subtraction,multiplication,division. However operations are completed in ascending order (via BODMAS). It would be useful to allow the use of brackets so that the order of operations can be chosen. This is especially important for the calculation of square root of a sum of channels.

[eagoetz]

The math operations are found here: https://github.com/gwpy/gwsumm/blob/master/gwsumm/data/mathutils.py