Improved bead mix functionality

Often, magbead buffers are quite viscous and/or dense relative to the buffers with which they are meant to mix.

Here, I've added the mix_lift argument to the bead_mix function to allow specification of tip lift behavior during mixing. mix_lift should be an int or float specifying the number of mm to lift the tip after aspirating and before dispensing in the mix step. mix_lift is added to z_offset, which specifies the height in mm above the bottom of the tube from which to aspirate.

Minor behavior bugfix: test error and touch tip on transfer

For viscous reagent transfers, such as washing with PEG, the add_buffer command should be able to invoke the touch_tip behavior.

Also fixed an erroneously written test.

More bugs

Fixed a bug in the buffer addition logic that could result in early shift to additional source columns when a transfer would result in a remaining volume == dead volume.

omg bugssss

prerelease-v0.2.3

bumped version for pip

Bugfix

prerelease-v0.2.2

added requests to install

Dev -- testing bead mix fixes

prerelease-v0.2.1

pulled master

Additional functions

Added magplate-specific bead elution function and robot renaming function.

Prerelease v0.2.0

Forgot to update setup.py for pip

Prerelease v0.1.0

This is the initial prerelease, suitable for installation and testing on an Opentrons OT-2 robot.