Update optimis(z)er interface to TF2

As described by @goldingn in #306 - here are some of the changes

This issue is about planning ahead to dealing with major TensorFlow API changes when a greta version has to work with TensorFlow 2.0. It doesn't affect anything yet.

optimisers

Tensorflow's various optimiser interfaces are being unified (details here). Some of the old versions will still be available in the compat compatibility module (though there may be speed benefit to changing to more recent versions). In addition, the TF 1.x contrib module will no longer be available in TF 2.0, and won't be in compat.

There is a tensorflow addons repo with an 'optimizers' module (though with no apparent overlap with existing optimisers) and a TFP 'optimizer' module which has a couple of the methods.

greta optimiser	TF 1.x function	TF2.0 function
`nelder_mead`	`tf$contrib$opt$ScipyOptimizerInterface`	`tfp$optimizer$nelder_mead_minimize`
`powell`	`tf$contrib$opt$ScipyOptimizerInterface`	?
`cg`	`tf$contrib$opt$ScipyOptimizerInterface`	?
`bfgs`	`tf$contrib$opt$ScipyOptimizerInterface`	`tfp$optimizer$bfgs_minimize`
`newton_cg`	`tf$contrib$opt$ScipyOptimizerInterface`	?
`l_bfgs_b`	`tf$contrib$opt$ScipyOptimizerInterface`	?
`tnc`	`tf$contrib$opt$ScipyOptimizerInterface`	?
`cobyla`	`tf$contrib$opt$ScipyOptimizerInterface`	?
`slsqp`	`tf$contrib$opt$ScipyOptimizerInterface`	?
`gradient_descent`	`tf$compat$v1$train$GradientDescentOptimizer`	`tf$keras$optimizers$SGD`
`adadelta`	`tf$compat$v1$train$AdadeltaOptimizer`	`tf$keras$optimizers$Adadelta`
`adagrad`	`tf$compat$v1$train$AdagradOptimizer`	`tf$keras$optimizers$Adagrad`
`adagrad_da`	`tf$compat$v1$train$AdagradDAOptimizer`	`tf$compat$v1$train$AdagradDAOptimizer`
`momentum`	`tf$compat$v1$train$MomentumOptimizer`	`tf$keras$optimizers$SGD`
`adam`	`tf$compat$v1$train$AdamOptimizer`	`tf$keras$optimizers$Adam`
`ftrl`	`tf$compat$v1$train$FtrlOptimizer`	`tf$keras$optimizers$FTRL`
`proximal_gradient_descent`	`tf$compat$v1$train$ProximalGradientDescentOptimizer`	`tf$compat$v1$train$ProximalGradientDescentOptimizer`
`proximal_adagrad`	`tf$compat$v1$train$ProximalAdagradOptimizer`	`tf$compat$v1$train$ProximalAdagradOptimizer`
`rms_prop`	`tf$compat$v1$train$RMSPropOptimizer`	`tf$keras$optimizers$RMSProp` (though arguments may have changed, so `tf$compat$v1$train$RMSPropOptimizer` as a backup)

Unless another interface to the Scipy optimisers is developed, the optimisers without replacements will have to be removed from the API. It'll bee a little hard to deprecate them without knowing whether a replacement will be coming.

ode solvers

The greta.dynamics package's ode_solve() functionality also depends on contrib, using the tf$contrib$integrate$odeint and odeint_fixed methods. tfp$math$ode$Solver$solve may be a viable replacement.

greta-dev / greta

Update optimis(z)er interface to TF2 #551

optimisers

ode solvers