Minor polish before new release

[dkarrasch]

• extends documentation to the new features slicing
• extends documentation to the new "scalar multiplication"/matrix conversion
• more care w.r.t. potentially non-1-based indices
• merge the different _generic_XYZ_mul!s to a single _generic_map_mul! and use multiple dispatch instead

[codecov]

Codecov Report

Merging #182 (0957f05) into master (0a42082) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #182      +/-   ##
==========================================
- Coverage   98.61%   98.60%   -0.01%     
==========================================
  Files          16       16              
  Lines        1372     1365       -7     
==========================================
- Hits         1353     1346       -7     
  Misses         19       19              

Impacted Files	Coverage Δ
src/LinearMaps.jl	100.00% <100.00%> (ø)
src/blockmap.jl	99.27% <100.00%> (ø)
src/conversion.jl	100.00% <100.00%> (ø)
src/transpose.jl	100.00% <100.00%> (ø)

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