nicer node printing & summary methods

[goldingn]

Printing nodes should say that they are greta nodes and preferably state the node type.

For data nodes, which have fixed values, they should summarise the data as if they were arrays (but with drop = FALSE). For stochastic or op nodes, they should summarise the data shape and print that with ? entries.

This gist defines an array-like class for unknown values, with subsetting methods etc. This can be used to overload head() for nodes. Printing would look like this:

> head(x)

greta node with unknown values

     [,1] [,2] [,3]
[1,]   ?    ?    ? 
[2,]   ?    ?    ? 
[3,]   ?    ?    ? 
[4,]   ?    ?    ? 
[5,]   ?    ?    ? 
[6,]   ?    ?    ? 

A dim() method (just returning x$dim), would also be helpful.

summary() and str() methods for distribution nodes should also return some information about the distributions and possibly parameters.

[goldingn]

'Node' isn't the best name for these (and it's confusing as nodes are defined internally, too). 'Tensor' may also be confused with TF tensors. Since they are intended to behave like arrays, perhaps 'greta array' is a better term?

The print method might then give:

greta array (fixed)

     [,1] [,2] [,3]
[1,]  0    1    0 
[2,]  1    0    1.2 

greta array (deterministic)

     [,1] [,2] [,3]
[1,]   ?    ?    ? 
[2,]   ?    ?    ? 

greta array (stochastic with beta distribution)

     [,1] [,2] [,3]
[1,]   ?    ?    ? 
[2,]   ?    ?    ? 

for consistency, and to hide internal functions from users, it might be better to thinly wrap these in a greta_array S3 class?

[goldingn]

ideas for the greta_array class are in this gist

[goldingn]

implemented in #24