Disabled the model cache

Somewhere is a bug that nobody is able to reproduce. Slow is better than wrong.
Arrow head in the components diagram now scales properly

A value in pixels was added to width of the arrow head to make it
bigger. This changes the proportions of the arrow head when the
diagram is zoomed. Always do relative calculations base on the bounds
that are passed in.

Now a factor is used to increase the width.
fixing the ValExpression to be usable only from within conversions
enhanced convert expressions
refactored conversion rule specification
fixed units tests
fixed units tests
fixed migration scripts
fixed an issue with val exp's unit
fixing the tutorial
phys units devkit setup
minor enhancements for the units editor
added missing dep
Merge remote-tracking branch 'origin/master'

Conflicts:
	code/languages/com.mbeddr.ext/tests/test.ts.ext.units/models/test/ts/ext/units/main@tests.mps
Merge branch 'master' of https://github.com/mbeddr/mbeddr.core

Conflicts:
	code/languages/com.mbeddr.ext/tests/test.ts.ext.units/models/test/ts/ext/units/main@tests.mps
removing legacy documentation about installing yices and nusmv
fixing the const in typedef original
improvement of the random models generator
small fixes in models-generator
improving type-system rules for stacks and ring-buffers