Fix many fixed

[clarabicalho]

Issue #246: Designers were retuning error when all arguments were fixed for some designers. For example:

pretest_posttest_designer(args_to_fix = names(formals(pretest_posttest_designer)))

Error in parse(text = x) : :1:131: unexpected symbol 1: substitute({report <- declare_assignment(prob = 1 - attrition_rate, assignment_variable = R) reveal_t2

PR

• Fixes issue above
• Adds tests for whether all designers work when all arguments are fixed

Note

Current use of substitute does not preserve comments in line. It only preserves comment lines that are separated from previous and subsequent code lines by a new line. For example, all of the below are preserved:

# D: Data Strategy 1
    select_case <- declare_sampling(
      strata = paste(X, Y),
      strata_n = c("X0Y0" = 0, "X0Y1" = 0, "X1Y0" = 0, "X1Y1" = 1))
    # I: Inquiry
    estimand <-
      declare_estimand(did_X_cause_Y = causal_process == 'X_causes_Y')
    # D: Data Strategy 2
    # Calculate bivariate probabilities given correlation

    joint_prob <- function(p1, p2, rho) {
...

But not the following:

population <- declare_population(
      N = N,
      X = rbinom(N, 1, prob_X) == 1,
      Y = (X & causal_process == "X_causes_Y") | # 1. X causes Y
        (!X & causal_process == "X_causes_not_Y") | # 2. Not X causes Y
        (causal_process == "Y_regardless") # 3. Y happens irrespective of X
    )

[clarabicalho]

closes #246