Source terms improvements

[timofeymukha]

It would be good to add these 2 things to source terms:

1. Possibility to write the source term as a field, for inspection. Useful in various debugging contexts.
2. Limit the addition of the source term in time. Useful for some flow cases, e.g. the "suddenly decelerated" channel flow.

Should be easy to implement, particularly after merging #1117