shape of design matrix needs to not be square necessarily

[tknapen]

Spatial dimensions of the design matrix should possibly be rectangular. this would

1. possibly be faster bc smaller design matrices
2. make it explicit which is the first and which is the second dimension

[tknapen]

This could be done by using pandas DataFrames for the design matrix, where columns and indices can tell us what x and y (for a 2D matrix) represent. Required changing the Stimulus class.