Declare missing dependencies

[rpanderson]

In #47 @philipstarkey identified some missing dependencies like zaber API, pyvisa, and OpalKelly (not on PyPI).

[dihm]

This was partially addressed when I updated the docs so RTD could actually build everything (see 45a54ed).

Still doesn't cover the list of non-pip installable ones, to which I would add PySpin and PyPylon.