[WIP] Fix Entity Table Order

[arnaudbore]

Following the table order it will reorder the key accordingly. Link.

We let some space for extensions but will put a warning if this happens (to do).

[SamGuay]

I added some additional warnings with the added flexibility.

Is a test required for this feature?

eg:

INFO:dcm2bids.sidecar:_mod-off_task-rest_echo-1_acq-1_weird-key_eeg_bold  <-  007_In_DCM2NIIX_regression_test_20180918114023
WARNING:dcm2bids.structure:Entity 'weird' is not a valid BIDS entity.
WARNING:dcm2bids.structure:There was more than one suffix found (['eeg', 'bold']), this is not BIDS compliant. Make sure you know what you are doing.
WARNING:dcm2bids.structure:✅ Filename was reordered according to BIDS entity table order:
                from:   sub-ID01_ses-001_mod-off_task-rest_echo-1_acq-1_weird-key_eeg_bold
                to:     sub-ID01_ses-001_task-rest_acq-1_mod-off_echo-1_weird-key_eeg_bold

[SamGuay]

Little cosmetic changes, everything works on my hand.