LIMS-1441 An error message related to partitions unit is shown when selecting analysis during AR creation

mL is not recognized as a unit by Magnitude, the module used by Bika LIMS for computing with numbers with units. Since mL is commonly used instead of ml to avoid confusion with the number one, "L" has been added as a 'recognizable' unit by Bika-LIMS. L unit (as liter) is also recommended by the [NIST Guide](http://physics.nist.gov/Pubs/SP811/sec05.html#table6).

For further reference, extract from the [Magnitude documentation|http://juanreyero.com/open/magnitude]:

```
A physical quantity is a number with a unit, like 10 km/h. Units can be any
of the SI units, plus a bunch of non-SI, bits, dollars, and any combination
of them. They can include the standard SI prefixes. Magnitude can operate
with physical quantities, parse their units, and print them. You don't have
to worry about unit consistency or conversions; everything is handled
transparently. By default output is done in basic SI units, but you can
specify any output unit, as long as it can be reduced to the basic units of
the physical quantity.

The basic units understood by the magnitude module are:

    indicator    meaning
    ---------    -------
    $            dollar ('dollar' is also acceptable)
    A            amperes
    bit          binary digit
    cd           candela
    K            degrees Kelvin
    kg           kilograms
    m            meters
    mol          amount of substance
    s            seconds

From these basic units you can derive many other units.  The magnitude
package predefines these derived units:

    Bq           becquerel
    C            coulomb
    c            speed of light (m/s)
    day
    degC         degree Celsius
    dpi          dots per inch
    F            farad
    ft           feet ("'" is also acceptable)
    g            gram
    gravity      acceleration due to gravity (m/s**2)
    Gy           gray
    H            henry
    h            hour
    Hz           Hertz
    inch         ('"' is also acceptable)
    ips          inches per second
    J            joule
    kat          byte
    l            liter
    lightyear    light year
    lm           lumen
    lpi          lines per inch
    lux
    min          minute
    N            newton
    ohm
    Pa           pascal
    S            siemens
    Sv           sievert
    T            tesla
    V            volt
    W            watt
    Wb           weber
    year

Two magnitudes have no units, 'rad' (radian - unit of plane angle) and 'sr'
(steradian - unit of solid angle).

Any of the above units can be augmented with the following set of scale
prefixes:

    letter     scale    name
    ------     -----    ----
    y          1e-24    yocto
    z          1e-21    zepto
    a          1e-18    atto
    f          1e-15    femto
    p          1e-12    pico
    n          1e-9     nano
    u          1e-6     micro
    m          1e-3     mili
    c          1e-2     centi
    d          1e-1     deci
    k          1e3      kilo
    Ki         2^10     Kibi
    M          1e6      mega
    Mi         2^20     Mebi
    G          1e9      giga
    Gi         2^30     Gibi
    T          1e12     tera
    Ti         2^40     Tebi
    P          1e15     peta
    Pi         2^50     Pebi
    E          1e18     exa
    Ei         2^60     Exbi
    Z          1e21     zetta
    Y          1e24     yotta
```