Calculating A260 units for loading polysome gradients

Fixing calculation of A260 when using less than ml in spec.ipynb

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    "Paraphrasing from [the Molecular Biology Problem Solver: A Laboratory Guide by Alan S. Gerstein (2001)](https://books.google.com/books?id=Jv-3O9rL6RUC&pg=PA276&lpg=PA276&dq=absorbance+unit+definition+ml+cm&source=bl&ots=geCgxkfKSD&sig=tIOhdxLA8E8b7Bu-xNwoWVRHvdM&hl=en&sa=X&ved=0ahUKEwjpp5Xf5a_PAhVDMyYKHVe4CIw4ChDoAQhLMAg#v=onepage&q=absorbance%20unit%20definition%20ml%20cm&f=false) and [here](http://ask.trilinkbiotech.com/q-what-are-od260-units/) and [here](https://www.idtdna.com/calc/Analyzer/Home/Definitions), one absorbance unit at the wavelength 260 nm (1 A$_{260}$ unit) is the amount of material required to give an absorbance reading of 1.0 at 260 nm in 1.0 mL of solution using a 1 cm light path. (This is somewhat of molecular biologist's operative term since strictly speaking absorbance is unitless, the [Molecular Biology Problem Solver: A Laboratory Guide by Alan S. Gerstein (2001)](https://books.google.com/books?id=Jv-3O9rL6RUC&pg=PA276&lpg=PA276&dq=absorbance+unit+definition+ml+cm&source=bl&ots=geCgxkfKSD&sig=tIOhdxLA8E8b7Bu-xNwoWVRHvdM&hl=en&sa=X&ved=0ahUKEwjpp5Xf5a_PAhVDMyYKHVe4CIw4ChDoAQhLMAg#v=onepage&q=absorbance%20unit%20definition%20ml%20cm&f=false) for more details.)\n",
    "\n"
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    "And so if 5 microliters of a sample into a total of 100 microliters gives an A$_{260}$ reading of 1.2, then we'd expect if we had diluted to the full mL assumed in the operative definition that we'd have 0.12 A$_{260}$ units or 0.024 A$_{260}$ units/microliter solution.\n",
    "\n",
    "Say we wanted to determine what volume would constitute 8 A$_{260}$ units:\n",
    "\n",
    "$$8 \\ A_{260} \\text{ units}/ 0.024 \\ A_{260} \\text{ units per microliter solution} = 333.33 \\text{ microliters}$$\n",
    "\n",
    "Therefore, we'd need 333.33 microliters of that sample to obtain 8 A$_{260}$ units."
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    "Additional resources\n",
    "--------------------\n",
    "\n",
    "* http://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Spectroscopy/Electronic_Spectroscopy/Electronic_Spectroscopy_Basics/The_Beer-Lambert_Law\n"
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