Refactoring - consolidate is_X (no args methods) to properties

I had a bug calling a non-property ```variant_coordinate.is_symbolic:``` where it was always True as I left off the brackets and it's a test against a method existing rather than calling function

Totally my fault - I wrote and called the funciton - just trying to head off future bugs

There are 3 is_symbolic methods:

```
Sequence.is_symbolic # normal function
VariantCoordinate.is_symbolic # normal function
Variant.is_symbolic # property
```

Talk to @TheMadBug - I think it would be worth adopting a policy of all is_X methods that don't take args as being properties?