Add `SNPs.merge` method

[apriha]

This PR adds a SNPs.merge method and obsoletes the SNPsCollection object. For the most part, the merging functionality is the same, but there are several improvements. Notably:

• A list or tuple of SNPs objects can be merged into a SNPs object
• If there is a mismatch between builds of files being merged, the SNPs to merge will be remapped automatically
• Merging code is cleaner and facilitates future updates (e.g., how to handle discrepancies)
• Properties and dataframes of the SNPs objects are merged
• Results of merging each SNPs object are returned

Other features of this PR include:

• Identification of SNPs with discrepant positions when saving VCF (SNPs with position outside reference sequence)
• Updated README examples

[codecov-commenter]

Codecov Report

Merging #91 into develop will increase coverage by 1.59%. The diff coverage is 95.53%.

@@             Coverage Diff             @@
##           develop      #91      +/-   ##
===========================================
+ Coverage    92.07%   93.66%   +1.59%     
===========================================
  Files            9        8       -1     
  Lines         1375     1358      -17     
  Branches       258      250       -8     
===========================================
+ Hits          1266     1272       +6     
+ Misses          67       47      -20     
+ Partials        42       39       -3     

Impacted Files	Coverage Δ
src/snps/__init__.py	100.00% <ø> (ø)
src/snps/io/writer.py	86.95% <90.90%> (+0.71%)	:arrow_up:
src/snps/snps.py	93.89% <96.03%> (+1.88%)	:arrow_up:
src/snps/resources.py	94.73% <0.00%> (+0.40%)	:arrow_up:
src/snps/ensembl.py	70.73% <0.00%> (+39.02%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update bcfce6c...7790187. Read the comment docs.