Download size reporting

[multimeric]

• Added un-exported functions url_file_size and report_file_sizes, for calculating and reporting the file sizes of URLs, with a test.
• Added a call to report_file_sizes before we download anything, which reports to the user how much data is about to be downloaded (closes Prepare users for 36GB download #60). This is the only user facing change. See below for an example.
• Added zzz.R which was supposed to be in Persistent connections #44. This closes all cached connections when the package is unloaded.
• Minor cleanup of imports and code style

> metadata <- get_metadata()
ℹ Downloading 1 file, totalling 0.48 GB
ℹ Downloading https://object-store.rc.nectar.org.au/v1/AUTH_06d6e008e3e642da99d806ba3ea629c5/metadata/metadata.0.2.3.parquet to /stornext/Home/data/allstaff/m/milton.m/.cache/R/CuratedAtlasQueryR/metadata.0.2.3.parquet
  |===================================================================================================================================================================| 100%
> single_cell_counts = 
+ 	metadata |>
+     dplyr::filter(
+         ethnicity == "African" &
+         stringr::str_like(assay, "%10x%") &
+         tissue == "lung parenchyma" &
+         stringr::str_like(cell_type, "%CD4%")
+     ) |>
+     get_SingleCellExperiment()
ℹ Realising metadata.
ℹ Synchronising files
ℹ Downloading 2 files, totalling 0.17 GB
ℹ Downloading https://swift.rc.nectar.org.au/v1/AUTH_06d6e008e3e642da99d806ba3ea629c5/harmonised-human-atlas/original/bc380dae8b14313a870973697842878b/assays.h5 to /vast/scratch/users/milton.m/cache/R/CuratedAtlasQueryR/0.2/original/bc380dae8b14313a870973697842878b/assays.h5
ℹ Downloading https://swift.rc.nectar.org.au/v1/AUTH_06d6e008e3e642da99d806ba3ea629c5/harmonised-human-atlas/original/bc380dae8b14313a870973697842878b/se.rds to /vast/scratch/users/milton.m/cache/R/CuratedAtlasQueryR/0.2/original/bc380dae8b14313a870973697842878b/se.rds
ℹ Reading files.                                                    
ℹ Compiling Single Cell Experiment.