(Now issue #450, locked) Float -> Delta

[OAGr]

We currently call our discrete distributions, with mass at one point, "Float". They really should be called "Delta".
https://en.wikipedia.org/wiki/Dirac_delta_function

We could also say "Dirac delta", but that seems too wordy.

https://docs.webppl.org/en/master/distributions.html
See webppl:

Discrete distribution that assigns probability one to the single element in its support. This is only useful in special circumstances as sampling from Delta({v: val}) can be replaced with val itself.

[quinn-dougherty]

#450

Will lock discussion