Allow building a Core resource from String

[clintval]

I have a pipeline where I would like to parameterize the compute resources for one of the tasks using the CLI like this toy example:

@clp(description = "Demo!", group = classOf[Pipelines])
class ExamplePipeline(
  @arg(doc="The cores for task 1") val cores: Cores = Cores(6.0),
  @arg(doc="The memory for task 1") val memory: Memory = "2G"
) extends Pipeline {

  override def build(): Unit = {
    task1 = new ShellCommand("echo", "task1").requires(cores, memory)
    task2 = new ShellCommand("echo", "task2")
    root ==> (task1 :: task2)
  }
}

At runtime I get:

...
--cores=Cores                 The cores for task 1. [Default: 6.0].
--memory=Memory               The memory for task 1. [Default: 2G].

Argument 'cores' could not be constructed from string
    ...Could not build a 'Cores' from a string: 6: dagr.core.execsystem.Cores$.apply(java.lang.String)
java.lang.Class.getMethod(Class.java:1786)
com.fulcrumgenomics.commons.reflect.ReflectionUtil$$anonfun$$nestedInanonfun$buildUnitFromString$1$1.$anonfun$applyOrElse$1(ReflectionUtil.scala:407)
scala.util.Try$.apply(Try.scala:209)
com.fulcrumgenomics.commons.reflect.ReflectionUtil$$anonfun$$nestedInanonfun$buildUnitFromString$1$1.applyOrElse(ReflectionUtil.scala:401)
com.fulcrumgenomics.commons.reflect.ReflectionUtil$$anonfun$$nestedInanonfun$buildUnitFromString$1$1.applyOrElse(ReflectionUtil.scala:399)
scala.runtime.AbstractPartialFunction.apply(AbstractPartialFunction.scala:34)
scala.util.Failure.recover(Try.scala:230)
...

Would you be willing to allow for building Core resources from Strings?

EDIT: Fixed type of cores in toy example.

[codecov-io]

Codecov Report

Merging #347 into master will increase coverage by 0.09%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #347      +/-   ##
==========================================
+ Coverage   91.44%   91.53%   +0.09%     
==========================================
  Files          31       31              
  Lines        1145     1146       +1     
  Branches      115      108       -7     
==========================================
+ Hits         1047     1049       +2     
+ Misses         98       97       -1

Impacted Files	Coverage Δ
...src/main/scala/dagr/core/execsystem/Resource.scala	95.91% <100%> (+2.16%)	:arrow_up:

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[nh13]

@clintval in your example, I think val cores: Cores = 6.0 will still need to be val cores: Cores = Cores(6.0) as there's no implicit in scope to convert type Double to type Cores, but other than that, LGTM.