Read PDD standard deviation from a file. See #179.

Add command-line options "-pdd_sd_file", "-pdd_sd_period" and "-pdd_sd_reference_year". If a file is given, standard deviation of near-surface temperature (variable "air_temp_sd") is read in in a similar way to temperature and precipitation time series and reported as 2D diagnostic. Do not affect PDD computation at the moment.
Apply constant SD if no file provided. See #179.

If no "-pdd_sd_file" is provided, use constant standard deviation value from configuration parameter "pdd_std_dev". This should ensure backwards compatibility for users not interested in the new option.
Use variable SD in PDD computation. See #179.

Compute PDD using "-pdd_sd_file" if provided, and constant standard deviation from "pdd_std_dev" configuration flag otherwise.
Document new options -pdd_sd_file etc. See #179.
Allow PDD SD parameterization over ice. See #265.

Allow parameterization of standard deviation as a linear function of air temperature over ice-covered grid cells. Enable with pdd_std_dev_use_param (off by default), and control with pdd_std_dev_param_a and pdd_std_dev_param_b.
Allow the case of zero PDD SD. See #265.
Set number of PDD SD evaluations per year.

This effectively synchronizes evaluation of PDD standard deviation and climate forcing in time-series interpolations. Resolve "double free or corruption" errors upon use of variable standard deviation. See #179.
Document PDD SD parametrization. See #265.
Fix a couple of errors in the PDD code.
Merge branch 'pdd-stdev' into dev