Update pdf for v2.1.0

See merge request !110

Update NEWS.md and add ecFlow into SystemRequirements

See merge request !109

Stop the execution after removing the bigmemory object in Compute

See merge request !108

Develop metadata missing dat

See merge request !106

Develop largest dim length

See merge request !107

Add new params 'largest_dims_length' to determine if Start() should examine all the files to get the dim or not

Set the result of nchar() to 0 if it is integer(0)

See merge request !104

Add one 'if' condition to ensure the first file of each dataset exists. If not, metadata is assigned to NULL.

Fix metadata reading bug when certain dataset is missing

See merge request !105