Extra files interfere with "pip install"

[tessmero]

Currently the installation command in the readme produces errors like this:

error: unable to create file code for the Zaks et al model

I made a fork that is easier to install. The fork does not have the Zaks et al. and Cruz et al. models and papers. If those files need to be kept in this repository, just the installation command in the readme could be changed to pull from the fork:

pip install git+https://github.com/tessmero/Delta_Psi_Py.git --upgrade --no-cache-dir