Add support for colon seperated list of library homes

[ChrisspyB]

Allows setting multiple homes in a colon seperated string. e.g. ECKIT_HOME=/path/to/home1:/path/to/home2
This is useful for libraries which like to search for multiple configs, which may be in distinct directories. LocalPathName's tilde expansion ~eckit/file will match the first existing file from [/path/to/home1/file1, /path/to/home2/file]. If none exist, the first is used.

No change in behaviour is intended if a single home is set.

If LIB_HOME is not set, there is a change in behaviour in the tilde expansion:

• Before: eckit would search recursively upwards from the prefix directory all the way to the root directory.
  • e.g. ~eckit/file1 -> libraryprefix/file1, libraryprefix/../file1, libraryprefix/../../file1, libraryprefix/../../../file1... /file1
• Now: eckit will search the prefix directories and one and two levels above it, then go straight to the root. (exactly four directories)
  • e.g. ~eckit/file1 -> libraryprefix/file1, libraryprefix/../file1, libraryprefix/../../file1, libraryprefix/../../file1, /file1

note This does mean that LIB_HOME cannot be set to directories whose path contains a colon.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.33333% with 3 lines in your changes missing coverage. Please review.

Project coverage is 63.73%. Comparing base (3598aa9) to head (beff400).
Report is 11 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/eckit/system/Library.cc	87.50%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #140   +/-   ##
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  Coverage    63.72%   63.73%           
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  Files         1065     1065           
  Lines        55141    55161   +20     
  Branches      4084     4085    +1     
========================================
+ Hits         35139    35157   +18     
- Misses       20002    20004    +2     

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