Adds SampledDimension axis overload

[jgrewe]

with this it is possible to specifiy a start position, not only a start index

start index takes precedence over start position to maintain old behaviour. added some value checking too.

[coveralls]

Coverage increased (+0.01%) to 79.037% when pulling 666697a on jgrewe:axis into 130fab2 on G-Node:master.

[codecov-commenter]

Codecov Report

Merging #504 (666697a) into master (130fab2) will increase coverage by 0.01%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #504      +/-   ##
==========================================
+ Coverage   78.76%   78.77%   +0.01%     
==========================================
  Files          59       59              
  Lines        9342     9356      +14     
==========================================
+ Hits         7358     7370      +12     
- Misses       1984     1986       +2     

Impacted Files	Coverage Δ
nixio/test/test_dimensions.py	99.52% <87.50%> (-0.24%)	⬇️
nixio/dimensions.py	91.32% <90.00%> (-0.08%)	⬇️

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[lgtm-com]

This pull request introduces 1 alert when merging 866331f into 8dcb254 - view on LGTM.com

new alerts:

• 1 for Unused local variable

[jgrewe]

closing and reopening this to see what happens to appveyor...