microeco v0.4.0

1. Add the rep_fasta to store the representative sequences in the microtable object.
2. Add the tax4fun2 method for the functional prediction of prokaryotic community.
3. Filter the taxa in which the sum of abundance is zero in otu_table.
4. Check whether the phylogenetic tree is rooted in microtable class.
5. Add the R2 and significance test in cal_rda() of trans_env class.
6. Move file reading and converting functions to package file2meco.
7. Optimize the cal_abund() to make it more flexible and compatible with the mixed user-defined data structure in tax_table.
8. Support the abundance calculation for multiple map correspondance of tax_table in cal_abund().
9. Add sub-network extracting by selecting edge label in subset_network function of trans_network class.
10. Add cal_module function in trans_network class to support module classification in specialized sub-network.