Add M.Q.Convert.DoubleAsStringWithFormat

[tcNickolas]

Is your feature request related to a problem? Please describe.
I print arbitrary floating-point numbers that are arguments to the tests in the katas quite often, and printing them with the default ~15 digits of precision doesn't look particularly good, especially if there are multiple numbers and a narrow space to display the text. Here is the example from the published Single-Qubit Gates kata, task "Prepare Arbitrary State":

Describe the solution you'd like
Bring back the function M.Q.Convert.DoubleAsStringWithFormat that used to be available in the classic QDK.

Describe alternatives you've considered
I could modify some of the tests to use "neat" numbers, but that makes the code much more complicated unnecessarily, and sometimes I need specific numbers that I cannot round to 3-4 digits of precision.

[filipw]

I also ran into this problem.

I think in general it would be good to get some guidance on how display formatting of numerical values is supposed to be done in the new QDK.