Uniformize compression options for all variables

[ElodieFZ]

Issue #12

Compression options are now the same for all variables: zlib=True, complevel=compression_level For compression level usually used is 7.

The chunking is removed for all variables except the S2 L2A specific layers. Somehow the chunking makes the output file bigger.

For example for the S2 L1C 'reference' product:

• with chunking for all variables: 1.3Gb
• with no chunking at all: 544Mb
• with chunking only for reflectance variables: 723Mb
• with chunking for reflectance variables and masks: 976Mb

[ElodieFZ]

@hevgyrt I would rather have your review before merging has maybe the chunking was set for a very specific purpose?

Also, if we want to run faster, lowering the compression level can be an option. It also makes (as far as I understand) the reading of data afterwards faster. I do not know though how to quantify that. As a quick example, with the lowest compression level (1), the sample file grows 20% in size, but the nc creation takes 25% less time.