Fail when user tries to set the algorithm to a list longer than one

[jordansread]

see #414 for details, but for an example

my_algorithms <- list(`Area Grid Statistics (unweighted)` = "gov.usgs.cida.gdp.wps.algorithm.FeatureGridStatisticsAlgorithm", `Area Grid Statistics (weighted)` = "gov.usgs.cida.gdp.wps.algorithm.FeatureWeightedGridStatisticsAlgorithm")

knife <- webprocess()
algorithm(knife) <- my_algorithms
knife
An object of class "webprocess":
url: https://cida.usgs.gov/gdp/process/WebProcessingService 
algorithm: Area Grid Statistics (unweighted) Area Grid Statistics (weighted) 
web processing service version: 1.0.0 
process inputs: 
   SUMMARIZE_TIMESTEP: false
   SUMMARIZE_FEATURE_ATTRIBUTE: false
   DELIMITER: COMMA
   REQUIRE_FULL_COVERAGE: true
   STATISTICS: 
   GROUP_BY: 
wait: FALSE 
email: NA 

☝️ looks like both values are in the algorithm slot, but probably only the first is used for the actual request:

knife@algorithm
$`Area Grid Statistics (unweighted)`
[1] "gov.usgs.cida.gdp.wps.algorithm.FeatureGridStatisticsAlgorithm"

$`Area Grid Statistics (weighted)`
[1] "gov.usgs.cida.gdp.wps.algorithm.FeatureWeightedGridStatisticsAlgorithm"