bugfix: revert epoch type

[dakimura]

WHAT

• revert EPOCH datatype

WHY

• because the enum value is used in header files. Once a data type is added, we must not delete it to keep the compatibility of the file format.

[codecov-commenter]

Codecov Report

Merging #617 (eddf809) into master (8f2ad88) will decrease coverage by 0.03%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #617      +/-   ##
==========================================
- Coverage   48.24%   48.21%   -0.04%     
==========================================
  Files         136      136              
  Lines       12976    12976              
==========================================
- Hits         6260     6256       -4     
- Misses       6106     6108       +2     
- Partials      610      612       +2     

Impacted Files	Coverage Δ
utils/io/datatypes.go	57.97% <ø> (ø)
utils/io/enumelementtype_string.go	25.00% <ø> (ø)
metrics/du.go	73.91% <0.00%> (-17.40%)	:arrow_down: