Scala 2.12.4 upgrade + improve input type resolution

[keithschulze]

Upgrades Scala to latest stable version.

Also, automatically cast input parameters to correct type to reduce boilerplate on developers. Previously, running the following with blob.gif open would give a ScriptException:

// @Dataset img
println(s"Img has ${img.getChannels} channel/channels")

// output: javax.script.ScriptException: value getChannels is not a member of Object 
// in println(s"Img has ${img.getChannels} channel/s")

This is because input parameters are injected into the script Bindings (i.e., Map<String, Object>), where type information is lost. This could be fixed by explicitly casting img to Dataset e.g., img.asInstanceOf[Dataset].getChannels. After this change, this is no longer necessary since we cast for input parameters to correct type automatically (i.e., through callback in ScriptModule). So, the following works:

// @Dataset img
println(s"Img has ${img.getChannels} channel/channels")

// output: Img has 1 channel/channels

[imagejan]

I assume this was closed in favor of #7?