Powerbox

--------------------
**Bugfixes**

- Fixed a bug in which the output power spectrum was a factor of sqrt(2) off in normalisation

--------------------
**Bugfixes**

- Fixed output of ``create_discrete_sample`` when not randomising positions.

**Enhancements**

- New option to set bounds of discrete particles to (0, boxlength) rather than centring at 0.

--------------------
**Features**

- New ``LogNormalPowerBox`` class for creating log-normal fields

**Enhancements**

- Restructuring of code for more flexibility after creation. Now requires ``cached_property`` package.

--------------------
First working version. Only Gaussian fields working.