Migrate to Timflow

[JoerivanEngelen]

Timflow has been released, and further convergence of steady-state and transient elements will happen here, which is a great step forward. One other nice thing is that elements have names that are more explanatory to most users, so we do not have to invent our own names anymore. Things to do:

• Replace calls to timml and timflow with calls to timflow in the codebase
• Distribute timflow with gistim exe instead of timml & ttim
• Update names on buttons in plugin with timflow names
• Update script generator
• Remove validation for multi-layer transient models. #193

[deltamarnix]

I have created a branch that uses timflow instead of timml and ttim and it seems to work out of the box. I made this branch on top of the dev-env branch, so I first want to make sure that that branch is in before I create a new PR for it.

[JoerivanEngelen]

PR is in #192