[Diff since v0.7.2](https://github.com/ModiaSim/ModiaBase.jl/compare/v0.7.2...v0.7.3)

- Speed improvements for structural and symbolic algorithms.
- Added support for state events, time events and synchronous operators
  (positive(), Clock(), after(), pre(), previous(), hold(), initial(), terminal())
- Added support for mixed linear equation systems having Real and Boolean unknowns.
- Simplified code for linear equation systems (while-loop instead of for-loop).
- Added TimerOutputs @timeit instrumentation to the solution of linear equation systems.


**Merged pull requests:**
- Support for positive, Clock and sample. (#3) (@HildingElmqvist)