Formats a string from a floating-point number. Value is converted
to string with the number of decimal places equal to digits. Formatting
is independent from locale with point used as decimal separator.
Returns string in the floating-point format: [+-]([0-9]*[.])?[0-9]+

## Usage

    (require '[utilities.format :refer :all])

    (double-to-str 1.9 4)
    => "1.9000"

    (double-to-str -45.688 2)
    => "-45.69"

    (double-to-str +45.688 2)
    => "45.69"