Calculates the path length of an input test_volume by comparing it to a
linear model using internal data of pathlengths vs volume. ... To work out
the pathlength in a microplate assay, there are two options. Option 1:
Measure the pathlength each time you do an assay by taking A975-A900 for each
well as the kfactor(well), and calculating pathlength = kfactor(well) /
kfactor(1cm pathlength). The 1cm k-factors can be derived using
get_kfactor() function. Option 2 (used here): Create a standardised dataset
of pathlength vs volume by measuring the A975-A900 of defined volumes of
buffer, and calculating the pathlengths of each (the pathlength_water_data
dataset) and use this to create a linear model of pathlength vs volume.
Experimental data suggests that aqueous buffers give very similar pathlength
values for a given volume, so here we use data from water to approximate all
aqueous buffers.