speed up mm_model_by_ply, other refinements

[aappling-usgs]

also added a 'modal' option to mm_get_timestep. big thing is the speed up, though

[coveralls]

Coverage increased (+0.2%) to 60.948% when pulling 1d5c71e on aappling-usgs:develop into 687320a on USGS-R:develop.

[coveralls]

Coverage increased (+0.2%) to 60.948% when pulling ace805d on aappling-usgs:develop into 687320a on USGS-R:develop.

[aappling-usgs]

current AppVeyor issues in build 1.0.233, line 287:

Installing GitHub packages: GLEON/LakeMetabolizer
+ Rscript -e 'library(devtools); library(methods); options(repos=c('\''http://cran.rstudio.com'\'','\''http://owi.usgs.gov/R'\'')); install_github(commandArgs(TRUE), build_vignettes = FALSE)' GLEON/LakeMetabolizer
Downloading GitHub repo GLEON/LakeMetabolizer@master
from URL https://api.github.com/repos/GLEON/LakeMetabolizer/zipball/master
Installing LakeMetabolizer
Skipping 2 unavailable packages: plyr, rLakeAnalyzer
"c:/R/bin/i386/R" --no-site-file --no-environ --no-save --no-restore CMD  \
  INSTALL  \
  "C:/Users/appveyor/AppData/Local/Temp/1/RtmpGyKHgz/devtools4384ff7129e/GLEON-LakeMetabolizer-ad6f501"  \
  --library="c:/RLibrary" --install-tests 

ERROR: dependencies 'rLakeAnalyzer', 'plyr' are not available for package 'LakeMetabolizer'
* removing 'c:/RLibrary/LakeMetabolizer'
Error: Command failed (1)
Execution halted
Command exited with code 1

[aappling-usgs]

mostly made that new commit to kick of a new appveyor build now that i've updated USGS-R/r-appveyor and USGS-R/r-travis. also good to send travis to the new CRAN repo, i hope

[coveralls]

Coverage increased (+0.2%) to 60.948% when pulling 259d5d2 on aappling-usgs:develop into 687320a on USGS-R:develop.

[aappling-usgs]

huzzah!