Rename assertType into either validateType or isType

[kungfooman]

This may cause a somewhat bigger refactor, because I'm no longer happy about the name "assertType" - IMO it should be called either validateType or - even shorter - isType - "assert" implies it will throw on wrong types, but it doesn't.

And then I'm also contemplating to rename the validate*-functions into assert*-functions and actually make them throw type errors, so we can merge certain types into one longer type error string.

I specifically think about: customValidations-for-entity.mjs

Example:

What I want to see here is simply: Entity#constructor> customValidation failed: x is NaN

[kungfooman]

I pretty much see two ways to unify warnings:

1. throw new Error('x is NaN') in combination with try / catch
2. Giving each function a warn method, the "parent" will just append all the calls to this callback

I'm tending to (2) since (1) may cause performance issues. I can also imagine that (2) will reduce circular dependency a bit, which is very welcome aswell, tracked in #54 (because we can remove the warn import declarations)