Adding an expected sensitivity curve simulation

[slwatkins]

I've written some Python code for calculating a sensitivity estimate given some experimental parameters.

• Created a file in the rqpy/sim/ folder called _sens_est.py to hold the code
• Added class rqpy.sim.SensEst which includes the heavy lifting
• Added function rqpy.sim.calculate_substrate_mass for converting volume to mass for various materials
• Added a demo showing how to use the code
• Added the option to not normalize the cdf in rqpy.sim.pdf_sampling (in case the PDF is already properly normalized)
• Made small docstring changes to some the functions in the limit submodule

Resolves #94.