Issue 237 optimisation

[manulera]

• Adds tm_neb to pydna.tm (uses NEB's API). It calls it using browser-like headers, which hopefully will work most of the time.
• Adds the possibility to use an estimate function as first attempt to design the primer, then calls the actual function to ge the result. This is useful when using NEB, which is pretty slow. To use that:

primer_design(dseqr, target_tm=55, tm_func=tm_neb, estimate_function=tm_default)

[codecov]

Codecov Report

Attention: Patch coverage is 95.91837% with 2 lines in your changes missing coverage. Please review.

Project coverage is 93.92%. Comparing base (7747787) to head (7e4f306).
Report is 2 commits behind head on dev_bjorn.

Files	Patch %	Lines
src/pydna/tm.py	87.50%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           dev_bjorn     #250      +/-   ##
=============================================
+ Coverage      93.66%   93.92%   +0.25%     
=============================================
  Files             40       40              
  Lines           3867     4080     +213     
  Branches         576      666      +90     
=============================================
+ Hits            3622     3832     +210     
- Misses           196      200       +4     
+ Partials          49       48       -1     

Files	Coverage Δ
src/pydna/design.py	97.51% <100.00%> (+0.31%)	⬆️
src/pydna/tm.py	96.92% <87.50%> (-3.08%)	⬇️

... and 2 files with indirect coverage changes

[manulera]

Tests are missing here, I will add them. Don't merge yet!

[manulera]

@hiyama341 this is ready and tested, have a look at the description to see how to use it!

[manulera]

@BjornFJohansson please have a look, I think this can be safely merged