Custom grid

[Astrokiwi]

A few updates

• allow custom grid to be distorted, so you can feed in data from a simulation or observation and overlay the LIC distortion on top (as is used for polarisation plots in astrophysics observations, and for velocity fields in astrophysics simulations)

• modifying imports for consistency

• modified import for tensorflow to support older logic used in code

• allow earlier versions of CMake (version 4 previously required is not in standard apt-get management for some older versions of Ubuntu, but it compiles fine with 3.5)

(Note: this is my first pull request, hopefully I'm doing this right!)