add Zygote and pre-allocated gradient buffer support

• add ValueGradientBuffer for pre-allocated gradient storage (#34)
• remove taped ReverseDiff-based AD (#36)
• remove support for rejection-based unwindind of the stack (#39)
• add support for Zygote-based AD (#40, experimental)

minor API and error wrapper changes

1. get_transformation and get_parent are now deprecated, use .transformation and Base.parent instead

2. Errors when constructing Value and ValueGradient now have their own exception type InvalidLogDensityException.

3. LogDensityRejectErrors can be told which exception types to catch, defaulting to the above.

Add wrapper for rejecting errors.

IMPORTANT: support for forwarding properties in wrappers (eg .transformation) was removed.

printing for ReverseDiff AD wrapper

v0.5.1

Add print support for ReverseDiff AD.

Add ReverseDiff support

Add ReverseDiff support. (#23)

Add ReverseDiff support,fix #22, improve missing AD method message

fix REQUIRE

v0.4.1

Fix REQUIRE.

API unification for AD

Merge pull request #21 from tpapp/tp/minor-enchancements

Tp/minor enchancements

fix UUIDs and REQUIRE

v0.3.0

fix REQUIRE

add AD via Flux

v0.2.0

fix transformvariables UUID

initial release

Merge pull request #14 from tpapp/tp/reorg-forwarddiff

move forwarddiff code to separate file