throw warning when encountering a folder while reading ogs5 input files.

An error is thrown when reading all ogs5 input files from a directory that contains a subfolder.

> ogs5_obj <- input_add_blocs_from_file(ogs5_obj = ogs5_obj,
+                                       sim_basename = "Ashi",
+                                       filename = "all",
+                                       file_dir = "examples/groundwater")
[1] "Reading file Ashi.bc"
[1] "Reading file Ashi.gli"
[1] "Reading file Ashi.ic"
[1] "Reading file Ashi.mmp"
[1] "Reading file Ashi.msh"
[1] "Reading file Ashi.num"
[1] "Reading file Ashi.out"
[1] "Reading file Ashi.pcs"
[1] "Reading file Ashi.st"
[1] "Reading file Ashi.tim"
[1] "Reading file Ashi_River.txt"
[1] "Reading file Ashi.log"
[1] "Reading file ensembles"
Error in file(file, "r") : cannot open the connection
In addition: Warning messages:
1: In file(file, "r") :
  'raw = FALSE' but 'examples/groundwater/ensembles' is not a regular file
2: In file(file, "r") :
  cannot open file 'examples/groundwater/ensembles': it is a directory
[1] "Reading file gwf.log"
[1] "Reading file Recharge.txt"
[1] "Reading file Wells.txt"

This error should be turned into a warning.