Add processing-mode and disposition-mode to the avhrr-l1b-eps file name

[TAlonglong]

The avhrr-l1b-eps file name is hardcoded with processing-mode to N and disposition-mode to O

During the commisioning of Metop-C at least the disposition-mode needs to added.

According to documentation: http://www.eumetsat.int/website/wcm/idc/idcplg?IdcService=GET_FILE&dDocName=PDF_AVHRR_L1B_PRODUCT_GUIDE&RevisionSelectionMethod=LatestReleased&Rendition=Web

Enumeration DISPOSITION_MODE
Value Name
T Testing
O Operational
C Commissioning

Enumeration PROCESSING_MODE
Value Name Description
N Nominal NRT processing
B Backlog Processing
R Reprocessing
V Validation

[coveralls]

Coverage decreased (-0.02%) to 78.282% when pulling 5d18c7f on TAlonglong:feature-handle-avhrr-l1b-eps-disposition-mode into ccacdf4 on pytroll:master.

[codecov]

Codecov Report

Merging #638 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #638   +/-   ##
======================================
  Coverage    78.3%   78.3%           
======================================
  Files         136     136           
  Lines       19735   19735           
======================================
  Hits        15453   15453           
  Misses       4282    4282

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[mraspaud]

LGTM

[mraspaud]

Thanks for the PR!