Add optional initializer! function to LazyBufferCache

[SouthEndMusic]

Fixes #110

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

[hexaeder]

Nice addition, but felt a bit breaking since it added a new type parameter, which is necessary for inferring get_tmp calls...
No hard feelings, easy enough to fix. Just commenting to raise awareness :)

[thomvet]

As a reference, an earlier discourse discussion on this topic: https://discourse.julialang.org/t/is-adding-a-type-parameter-to-a-struct-a-breaking-change/111236

The change implemented here concerned the implementation of the type (new type parameter), but the interface change was implemented in a non-breaking way (default argument to constructor that kept behavior the same).

One can argue that fields and parameters of types are ultimately implementation details and that guarantees apply only to the interface in which the types are used. That would make this technically a non-breaking change, even though it obviously broke your code in this instance.

[ChrisRackauckas]

Julia's dispatch is specifically designed so that new type parameters can be omitted so that adding new parameters does not break existing code.