Need a mechanism for managing python dependencies

[skugele]

As a lidapy developer, I need the ability to add and manage additional python packages to support new lidapy functionality.

Currently, the only lidapy dependencies are being installed by apt-get because they are part of larger non-python dependencies (e.g., ROS). The best way to manage python specific dependencies seems to be pip.

I propose that we create a requirements.txt file to document our python specific dependencies, and allow them to be installed automatically using pip install -r requirements.txt.

As a first example of this new mechanism, I intend on using it to manage a dependence on the mock package in support of unit testing. (This is available in version of Python3 in the unittest package, but is not available in the core Python2 packages.)

Assuming this works, I will document this in the README.md as an installation step, and update the Travis CI testing script to run pip install -r requirements.txt.