Add sample_std parameter to RandGaussianNoise

[bakert1]

Is your feature request related to a problem? Please describe.
The documentation for RandGaussianNoise does not make clear that the standard deviation is sampled uniformly from 0 to std. (at least in my opinion; the doc-string of std reads "Standard deviation (spread) of distribution'). Additionally, it may be useful to have the option to not sample the standard deviation, and just use std as the spread every time.

Describe the solution you'd like
Add a sample_std parameter to RandGaussianNoise. When True, the noise's standard deviation is sampled uniformly from 0 to std (i.e., what is currently done). When False, the noise's standard deviation is non-random and set to std. The default for sample_std would be True for backwards compatibility.

Describe alternatives you've considered
Updating the documentation of RandGaussianNoise to make more clear that the std is being sampled.

Additional context

• Possible use case of sample_std=False: Adding different levels of random Gaussian noise to images to test the robustness of a registration algorithm to varying levels of Gaussian noise.
• RandRicianNoise is similar to RandGaussianNoise in concept and implementation. RandRicianNoise already has the proposed sample_std parameter.
• I'd like to implement this feature request and think it would be a good first issue. I would base the implementation off of RandRicianNoise's implementation.