Fix #93 Rewrite FiniteMPS constructors

[lkdvos]

This updates the existing constructors, such that all specifications of virtual spaces are now considered to be an upper bound, instead of a fixed virtual space. This has the following advantages:

• Users can no longer accidentally create non-full rank MPS
• Alternating virtual space specifications can now be achieved easily with maxvspaces = repeat([V1, V2], N/2)

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (c6a52bb) 84.22% compared to head (a9fc123) 84.27%.

Files	Patch %	Lines
src/states/finitemps.jl	85.71%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #102      +/-   ##
==========================================
+ Coverage   84.22%   84.27%   +0.04%     
==========================================
  Files          63       63              
  Lines        3849     3848       -1     
==========================================
+ Hits         3242     3243       +1     
+ Misses        607      605       -2     

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[DaanMaertens]

Looks good. I think it is ready to merge.