Improve file examples

[eloch216]

Some users had reported issues when reading files using read_gasex_file. After looking in to these issues, it was found that these users were directly copying the example code like

file_paths <- c(
  system.file('extdata', 'ball_berry_1.xlsx', package = 'PhotoGEA'),
  system.file('extdata', 'ball_berry_2.xlsx', package = 'PhotoGEA')
)

and replacing ball_berry_1.xslx and ball_berry_2.xlsx with their own file names.

This had a very confusing and unexpected result. First, system.file returned an empty string because it could not find the specified file. Then, read_gasex_file interpreted the empty string as if it were a plaintext Licor Li-6800 log file name. Eventually it encountered a problem when calling read.delim, and reported an unhelpful error message like Error in if (skip > 0L) readLines(file, skip) : argument is of length zero.

This PR addresses this problem in several ways:

• read_gasex_file now returns a helpful error message if the file_name input is an empty string.
• The examples involving system.file now all specify mustExist = TRUE. With this option, an error is generated if the file cannot be found, instead of the default behavior of returning an empty string.
• The vignettes now clarify that system.file should not be used for loading the user's own files.

Hopefully now users will no longer copy the system.file command. If they do, they will probably also copy the mustExist = TRUE option, which will cause a more direct error message explaining that the file cannot be found. If a user somehow persists in passing an empty string to read_gasex_file, it will also produce a more informative error message.