Add tqdm progress bar to dials.import (cctbx#768)

Give the user some hint as to the progress i.e. the computer is doing
something when you import thousands of files.

Only show progress bar if > 1 file and isatty, includes a cast of the `filenames` iterator to a list
to allow evaluation of the length for progress assessment.

Resolves dials/dials#2783

newsfragments/768.feature

@@ -0,0 +1,2 @@
+``dials.import``: add a progress bar for the import process, particularly
+useful when importing thousands of files

src/dxtbx/model/experiment_list.py

@@ -14,6 +14,7 @@
 from typing import Any, Callable, Generator, Iterable
 
 import natsort
+from tqdm import tqdm
 
 import dxtbx
 from dxtbx.format.Format import Format
@@ -664,14 +665,23 @@ def from_filenames(
         """Create a list of data blocks from a list of directory or file names."""
         experiments = ExperimentList()
 
+        # Cast filenames to a list from whatever iterator they are
+        filenames = list(filenames)
+
         # Process each file given by this path list
         to_process = _openingpathiterator(filenames)
         find_format = FormatChecker()
 
         format_groups = collections.defaultdict(list)
         if format_kwargs is None:
             format_kwargs = {}
-        for filename in to_process:
+
+        if os.isatty and len(filenames) > 1:
+            filename_iter = tqdm(to_process, total=len(filenames))
+        else:
+            filename_iter = to_process
+
+        for filename in filename_iter:
             # We now have a file, pre-opened by Format.open_file (therefore
             # cached). Determine its type, and prepare to put into a group
             format_class = find_format.find_format(filename)