It should be possible to customise the axis and pipetting speed of the OT2

[Brad0440]

As noted in the comment below, it should be possible for users to modify the speed that the OT-2 works at, in terms of both head movement and pipetting speed.

It should also be possible to customise the movement speed of the pipetting head in general. Similar to how it is possible to define the starting tip position.
Originally posted by @davidcmarkham in #20 (comment)

This would probably be best implemented as a set of functions (XY_Speed, Aspirate_Speed, Z_Speed, Mixing_Speed, etc.), and also added as attributes and methods to the OT2 template superclass.