Random sink frequency

[nuclearkatie]

Adds randomness in the next time Sink requests material through random_frequency_type. Options:

• UniformInt, which also requires random_frequency_max (otherwise the default is 1e299, making it extremely unlikely that the facility ever gets called), and optional random_frequency_min, which has a default of 1
• NormalInt which has random_frequency_mean and random_frequency_stddev, and can will use random_frequency_min and random_frequency_max when provided

Tests

• UniformInt
• NormalInt with default parameters
• NormalInt with specified mean/stddev
• Using randomness in size (from add random behavior for size of sink request #550) and frequency at the same time

Closes #551 (Along with #550, which is already merged) and closes #559

[gonuke]

This looks mostly ok. I'd like to little more comments in the tests so that we know why they are right. (I believe that they are right, but want to leave comments for future us.)

[gonuke]

Excellent - thanks for these comments.