1. Convert several serial loops to do concurrent.
2. Collapse array statements.
3. Associate runtime constants.
4. Use garden's assert_equals_within_absolute.
5. Move sim_speeds logic to internal pure function.

use the count() intrinsic function to convert the distribution
calculation into a concurrent loop

This commit collapses the vel calcuation to 1 array statement.

This commit defines a new output_t typ and moves the
build_distribution type-bound function to this new type,
where it can get the empirical distribution from the copy
of the input stored as a component of output_t.

1. Rename function build_distribution -> simulated_distribution.
2. Let simulated_distribution use the output_t history_ component.

This eliminates all arguments required by this function except
the passed-object dummy argument "self".