Adding function constant_sig to allow simulations with constant standard deviation. Updating tests, docs, and version number accordingly.

- Add get_nt method to TimeConstraint
- Change gen_coh_file for new api
- Update tests to reflect new api

Copy of Emmanuel's work in !45, local branch to allow rebases.

Resolve "Missing logic on data functions when no constraint in that direction"

Closes #27

See merge request !53

updating version for 2.6.1 release

See merge request !52